Diseases: Types, Transmission and Vaccines

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Context: Recently, to deal with zoonotic diseases, a need to operationalize the “OneHealth’ policy in India was highlighted. Presently, India is dealing with coronavirus (COVID-19). The virus appears to have its origin in bats.

Prelims: Current events of national and international importance.
Mains: GS II-

  • Issues relating to the development and management of Social Sector/Services relating to Health, Education, Human Resources.

  • Health is described as the state of complete physical, mental and social well being. Being healthy is far more than just being free from disease.
  • The disease is a condition of disturbed functioning of the body caused by infection, defective diet, heredity, environment or deprived condition of the brain.
  • Health is a state of complete physical, mental and social well being.

The disease may be a response to

  • Environmental factors (as malnutrition, industrial hazards or climate)
  • Specific infective agents (as worms, protozoans, fungi, etc)
  • Inherent defects of the organism (as genetic anomalies.)
  • Combination of these factors
Causes of Diseases/Disease Agents
  • Disease agent is organism, substance or force which causes disease due to its excessive presence, deficiency or absence.
    • Pathogens/Biological Agents:
      • They are biological entities which cause infectious diseases, e.g., viruses (mumps, chickenpox, smallpox), mycoplasma (e.g, bronchitis, acute leukemia), chlamydia (e.g, trachoma), bacteria (e.g. cholera, tetanus), fungi (ringworm, thrush, moniliasis, pulmonary aspergillosis), protozoa (e.g. giardiasis, sleeping sickness), helminths (e.g., filariasis, ascariasis, taeniasis), other organisms (e.g., scabies).
    • Nutrient Agents:
      • Deficiency of vitamins (e.g., beriberi, scurvy, night blindness), minerals (e.g., anemia, rickets), carbohydrates, fat and proteins (e.g., kwashiorkor, marasmus), or excess of food (e.g., obesity).
    • Chemical Agents:
      • Endogenous Agents, Excess presence of uric acid, reduced secretion of ADH (diabetes insipidus) or insulin (diabetes mellitus). Exogenous Agents- Pollutants (e.g., pneumoconiosis), allergens (allergy).
    • Physical Agents:
      • Heat (e.g., stroke), cold (frostbite), radiations, sound (impaired hearing), humidity, etc.
    • Mechanical Agents:
      • Fractures, sprains, dislocation, injury, chronic friction.
    • Genetic Agents:
      • Excess or deficiency of chromosomes, mutations, harmful alleles, e.g, colour blindness, albinism, hemophilia, Turner’s syndrome.
    • Degeneration:
      • They include old age changes like peptic ulcers, hypertension, atherosclerosis.
Types of Disease
  • 1.Congenital diseases:
    • Congenital disorders can be due to fault in the chromosome structure or damage inflicted on the developing embryo.
    • These could be caused by radiation, diseases contracted by the mother (German measles), use of certain drugs, excessive smoking and alcohol intake by the pregnant mother; for example, Hare-lip, club foot, and Mongolism.
  • 2.Hereditary Diseases:
    • Diseases that are transmitted from parent to offspring from generation to generation are termed hereditary diseases. E.g., Haemophilia and colour blindness.
  • 3.Acquired diseases:
    • Acquired diseases develop in an individual after birth. These are of two kinds:
      • (I) Communicable diseases:
        • These are caused due to the entry of disease-causing germs called pathogens into the body and are easily transmitted from person to person by direct or indirect contact or through a carrier which is called a vector, e.g., mosquito (Anopheles) is a vector of malaria. Indirect contact may be through clothes, beddings, utensils, etc.
        • Communicable diseases are further classified into several types depending on the types of causative agents:
          • (i) Viral (ii) Bacterial, (iii) Protozoan (iv) Helminthic (v) Fungal.
      • (II) Non-Communicable diseases:
        • These are restricted to the persons suffering and they are of the following types:
          • (a) Degenerative Diseases:
            • These diseases are due to the degeneration of tissues in old age, that is, diseases caused due to a decline in the ability of the body to repair its tissues.
            • It leads to malfunctioning of the heart, lungs and central nervous system E.g., Parkinson’s disease, cataract, and arteriosclerosis.
          • (b) Cancer:
            • It is caused due to uncontrolled growth of tissues in any part of the body.
            • This disease has become a challenge to medical science as it is incurable in later stages.
          • (c) Allergies:
            • These are caused due to hypersensitivity of the body to certain foreign substances called Allergens. E.g., Hay fever, asthma, nettle rash.

  • Bacteria are prokaryotes, a minuscule single-celled organism that grows well in varied environments.
  • They can live inside the soil, in the ocean, and inside the human bowel. They can be differentiated, by their shape, the nature of their cell walls and genetic differences.
  • The bacteria multiply by a process called binary fission.
Diseases Bacteria
  • Cholera
Vibrio cholerae
  • Leprosy
Mycobacterium Leprae
  • Plague
Yersinia pestis
  • Pneumonia
Streptococcus pneumoniae
  • Tetanus
Clostridium tetani
  • Tuberculosis
Mycobacterium tuberculosis.
  • Typhoid
Salmonella typhi
  • Whooping Cough
Bordetella pertussis.


  • The virus is a tiny infectious agent that duplicates only inside the living cells of other creatures.
  • These are diverse in nature and can infect animals, plants, and microorganisms and transmitted by biological vectors only.
  • It is made up of a DNA or RNA genome inside a protein shell known as the capsid. Some viruses have an internal or external membrane covering.
  • It lacks enzymes essential for energy production.
Diseases Virus
  • AIDS
Human Immunodeficiency Virus (HIV)
  • Chickenpox
Varicella-zoster virus.
  • Common Cold
  • Chikungunya
Chikungunya Virus
  • Dengue fever
Dengue virus
  • Tuberculosis
Mycobacterium tuberculosis.
  • Ebola
Ebola virus
  • Foot and mouth
Foot-and-mouth disease virus (FMDV)
  • Hepatitis A 
Hepatitis A Virus(HAV)
  • Measles
Measles virus
  • MERS
Coronavirus (MERS-CoV)
  • Mumps
Mumps virus.
  • Rabies
Rabies virus
  • Small Pox
Variola virus


  • Protozoa are a varied cluster of unicellular eukaryotic organisms that consume bacteria and other food sources.
  • The diseases caused by Protozoa are life-threatening and dangerous.
Diseases Protozoa
  • Amoebic dysentery
Entamoeba Histolytica.
  • Kala Azar
Leishmania Donovani.
  • Malaria
Plasmodium parasite
  • Sleeping sickness
Trypanosoma Brucei.


Zoonotic Diseases
  • Zoonosis refers to the transmission of diseases between animals and humans. Such diseases are termed as Zoonotic Diseases.
  • Zoonotic diseases range from mild to severe, while in extreme cases can even be fatal.
  • Zoonoses may be bacterial, viral, or parasitic, or may even involve unconventional agents for the transmission of the disease.
  • WHO in 1959 defined Zoonoses as “those diseases and infections which are naturally transmitted between vertebrate animals and man.”
  • The World Zoonoses Day is observed every year on July 6 to create awareness on zoonotic diseases, how to prevent them and what actions to take when exposed.  
  • Classification of Zoonotic diseases:
    • The classification of diseases is usually done on the basis of the pathogen causing the disease.
    • With the advancement in science, scientists and medical professionals have been able to identify and study in detail the etiological agents causing the diseases.
    • Etiological agents usually refer to the causative agent of a diseased condition. 
  • 1. Rotavirus
    • About:
      • Rotavirus is the most common virus that contaminates the bowels.
      • It causes diarrhea among infants and children throughout the world and causes over 450,000 deaths worldwide annually inclusive of 110,000 deaths in India which are accounts for 22 % of the estimated global deaths from diarrhea-causing rotavirus.
    • Symptoms:
      • Rotavirus affects the body in many ways and multiple infections can be noticed.
        • Vomiting
        • Low-grade Fever
        • Watery diarrhea
        • Nausea
    • What is the Rotavirus vaccine?
      • Rotavirus vaccine prevents diarrhea virus to enter the body which causes death
      • RotaShield is a Rotavirus vaccine by Wyeth was licensed in 1998 in the United States
      • In 1999, however, the manufacturer withdrew it from the market risking bowel obstruction in one of every 12,000 vaccinated infants
      • The vaccine was under trial for 8 years since the withdrawal.
      • After efficient research, Rotarix by GlaxoSmithKline and RotaTeq by Merck were manufactured which is effective and safe.

  • 2. Zika Virus
    • About:
      • Zika virus infection is caused by the bite of an infected Aedes mosquito.
      • It is spread by mosquitoes of the Aedes genus (Aedes aegypt) which can breed in a pool of water as small as a bottle cap and usually bite during the day.
      • The Asian tiger mosquito, Aedes albopictus, is also known to transmit the virus.
    • Symptoms and Complications :
      • Microcephaly and Guillain-Barré syndrome
      • Microcephaly is a condition where a baby’s head is much smaller than expected. It occurs among babies born to women who contracted the virus during pregnancy
      • Guillain-Barré syndrome is a rare condition in which the immune system attacks the peripheral nervous system. For some, GBS can be quite serious, causing muscle weakness, sensory problems, pain and sometimes, paralysis.

  • 3. Ebola Virus
    • About:
      • Ebola virus disease (EVD), formerly known as Ebola hemorrhagic fever, is a severe, often fatal illness in humans.
      • The virus is transmitted to people from wild animals and spreads in the human population through human-to-human transmission.
      • Fruit bats of the Pteropodidae family are considered to be the natural host of the Ebola virus.
      • Ebola is introduced into the human population through close contact with the blood, secretions, organs or other bodily fluids of infected animals.
    • Signs and symptoms:
      • EVD is a severe acute viral illness often characterized by the sudden onset of fever, intense weakness, muscle pain, headache, and sore throat.
      • This is followed by vomiting, diarrhea, rash, impaired kidney and liver function, and in some cases, both internal and external bleeding.
      • The incubation period, that is, the time interval from infection with the virus to onset of symptoms is 2 to 21 days.
      • In the absence of effective treatment and a human vaccine, raising awareness of the risk factors for Ebola infection and the protective measures individuals can take is the only way to reduce human infection and death.

  • 4.Middle East Respiratory Syndrome virus
    • MERS stands for Middle East Respiratory Syndrome virus. It was first seen in Saudi Arabia in 2012.
    • It’s a coronavirus – a family of viruses that usually cause common colds and that can infect both animals and people. It’s a relative of SARS, the Severe Acute Respiratory Syndrome virus that swept around the world, infecting more than 8,000 people globally and killing 774 before it was stopped in 2004.
    • MERS can cause fever, cough, shortness of breath, diarrhea, body aches, nausea and, in the most serious cases, pneumonia and kidney failure, according to the U.S. Centers for Disease Control and Prevention. Its incubation period is usually 5 or 6 days, but people can be contagious for up to 14.
    • The virus kills by causing respiratory or kidney failure, or septic shock, an infection that overwhelms the body’s defenses.
      • Severe acute respiratory syndrome (SARS):
        • It is believed that the 2003 SARS epidemic started when the virus spread from small mammals in China.
        • SARS symptoms often included fever, chills and body aches which usually progressed to severe pneumonia and breathing difficulty.
        • SARS is more deadly but much less infectious than COVID-19. There have been no outbreaks of SARS anywhere in the world since 2003.

  • 5. Swine Flu
    • About:
      • Swine flu (swine influenza) is a respiratory disease caused by viruses (influenza viruses) that infect the respiratory tract of pigs and result in nasal secretions, a barking cough, decreased appetite, and listless behavior.
      • The virus spreads by tiny droplets that are released when a person coughs or sneezes. The droplets reach a distance of about one meter (3ft). 
      • Common objects such as door handles, remote control, handrails, and computer keyboards can get contaminated with the virus when the droplet settles on these surfaces.
    • Signs and symptoms:
      • Fever (100 0F or greater), cough, nasal secretions, fatigue, and headache, with fatigue being reported in most infected individuals. Some patients also get nausea, vomiting, and diarrhea.
      • Some patients develop severe respiratory symptoms and need respiratory support (such as a ventilator to breathe for the patient).
      • In 2015 the states of Gujarat and Rajasthan are the worst affected by Swine Flu.

  • 6. Filariasis
    • Disease caused by roundworms with mosquitoes as the carrier.
    • Filariasis is a parasitic disease caused by an infection with roundworms.  It is also called as Elephantiasis or Hathipaon. India has a campaign – Hathipaon Mukt Bharat (Filaria Free India) for preventive medication.
    • The national health policy had aimed at eliminating filariasis by 2015. The deadline was extended to 2017 and now has been shifted to 2020.

  • 7.Malaria
    • Malaria is caused by plasmodium protozoa and is most commonly transmitted by an infected female Anopheles Mosquito.
    • The mosquito bite introduces the parasites from the mosquito’s saliva into a person’s blood. The parasites travel to the liver where they mature and reproduce.

  • 8.Kala-Azar or Black Fever or Dumdum Fever or Leishmaniasis
    • Protozoan disease with Sandfly as the carrier.
    • It is a slow progressing indigenous disease caused by a protozoan parasite of genus Leishmania. There is only one sandfly vector of Kala-azar in India.
    • India has missed the 2017 deadline that the Finance Minister had announced for the elimination of Kala Azar (black fever) in his Budget speech. In fact, endemic blocks have increased from 61 to 68 in 17 districts of Bihar and Jharkhand.
  •  9.Japanese Encephalitis
    • Viral disease with Culex Mosquito as the carrier.
    • The death of 63 children in Gorakhpur’s BRD Medical College is being attributed to JE.

Quick Reference for diseases transmitted by Mosquitos 
  • Aedes :
    • Chikungunya, Dengue fever, Lymphatic filariasis, Rift Valley fever, Yellow fever, Zika
  • Anopheles:
    • Malaria, Lymphatic filariasis
  • Culex:
    • Japanese encephalitis, Lymphatic filariasis, West Nile fever


Novel Coronavirus (Covid-19)
  • A new strain of the coronavirus that has not been identified previously is called a novel coronavirus (nCov).
  • Coronavirus disease 2019 is an infectious disease caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), a virus closely related to the SARS virus.
  • The 2019-nCov has been given the official name as COVID-19.

Coronavirus (COVID-19) – Origin and Transmission

  • Coronaviruses originate in animals like camels and bats and are zoonotic diseases as it passes from animals to humans occasionally.
  • The source of a zoonotic disease is called reservoir species. (For SARS, the reservoir species was identified as bats).
  • The first known case of COVID-19 was reported in the Chinese city of Wuhan in December 2019 and was traced to an animal market in the city.
  • Research is ongoing on how COVID-19 spreads. However, as per WHO, the disease spreads in humans via exposure to respiratory secretions – the small droplets from the nose or mouth of an infected person – either directly or indirectly.

The basic reproduction number (R_0)

  • It is a measure of transmissibility that aims to describe the average number of people a new case will infect. For COVID-19, WHO estimates R_0 = 1.4 to 2.5. However, many researchers think this figure is under-estimated.
  • Compared to earlier outbreaks of SARS and MERS, COVID-19 has a greater global spread and researchers think that mutation in the virus strain is enabling it to be more efficiently transmitted.


  • The vaccine is a biological preparation that improves immunity to a particular disease.
  • A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe or its toxins.
  • The agent stimulates the body’s immune system to recognize the agent as foreign, destroy it, and “remember” it, so that the immune system can more easily recognize and destroy any of these microorganisms if it encounters in the future.
Properties of ideal vaccine
  • Provide long-lasting immunity.
  • Should induce both humoral and cellular immunity.
  • Should not induce autoimmunity or hypersensitivity.
  • It should be inexpensive to produce, easy to store and administer.
  • Vaccines must also be perceived to be safe.
  • The vaccine vial may contain relevant antigen, adjuvant (usual alum), preservatives and/or traces of protein derived from the cells in which the vaccine agent was cultured e.g. egg protein
Types of vaccines
  • a) Inactivated vaccines:
    • When inactivated vaccines are made, the bacteria are completely killed using a chemical, usually formaldehyde.
    • Dead pieces of disease-causing microorganisms (usually bacteria) are put into the vaccine.
    • Because the antigens are dead, the strength of these vaccines tends to wear off over time, resulting in less long-lasting immunity.
    • So, multiple doses of inactivated vaccines are usually necessary to provide the best protection.
    • The benefit of inactivated vaccines is that there is zero chance of developing any disease-related symptoms — allergic reactions are possible but extremely rare.
    • Examples of inactivated vaccines are hepatitis A, hepatitis B, poliovirus, hemophilic influenza type b, meningococcal, pneumococcal and the injected form of influenza.
  • b) Live-attenuated vaccines:
    • Live-attenuated basically means alive, but very weak.
    • These vaccines are made when the virus is weakened to such a level that they reproduce only about 20 times in the body.
    • When the vaccine is made, the virus or bacteria is weakened in a laboratory to the point where it’s alive and able to reproduce, but can’t cause serious illness. Its presence is enough to cause the immune system to produce antibodies to fight off the particular disease in the future.
    • They typically provoke more durable immunological responses and are preferred for healthy adults.
    • Examples include the viral diseases yellow fever, measles, rubella, and mumps and the bacterial disease typhoid.
  • c) Recombinant Vector vaccine:
    • By combining the physiology of one micro-organism and the DNA of the other, immunity can be created against diseases that have complex infection processes.
    • There are four genetically-engineered vaccines are currently available:
      • Hepatitis B vaccines are produced by insertion of a segment of the hepatitis B virus gene into the gene of a yeast cell. The modified yeast cell produces pure hepatitis B surface antigen when it grows.
      • Human papillomavirus vaccines are produced by inserting genes for a viral coat protein into either yeast (as the hepatitis B vaccines) or into insect cell lines. Viral-like particles are produced and these indices a protective immune response.
      • Live typhoid vaccine (Ty21a) is Salmonella typhi bacteria that has been genetically modified to not cause illness.
      • Live attenuated influenza vaccine (LAIV) has been engineered to replicate effectively in the mucosa of the nasopharynx but not in the lungs.
  • d) DNA vaccination:
    • In recent years a new type of vaccine called DNA vaccination has been created from an infectious agent’s DNA.
    • As in complex diseases the DNA quality of the infection changes thus no vaccine works on it.
    • DNA vaccine works by insertion and expression, triggering immune system recognition of viral or bacterial DNA into human or animal cells.
    • Some cells of the immune system that recognize the proteins expressed will mount an attack against these proteins and cells expressing them.
  • e) Polysaccharide vaccines:
    • Polysaccharide vaccines are a unique type of inactivated subunit vaccine composed of long chains of sugar molecules that make up the surface capsule of certain bacteria.
    • Pure polysaccharide vaccines available include pneumococcal, meningococcal, and Salmonella typhi.
  • f) Anti-Idiotypic Vaccine:
    • An antigen-binding site in an antibody (paratope) is a reflection of the three-dimensional structure of part of the antigen (epitope).
    • This unique amino acid structure in the antibody is known as the idiotype, which can be considered as a mirror of the epitope in the antigen.
    • Antibodies can be raised against the idiotype by injecting the antibody into another animal.
    • This anti-idiotype antibody mimics part of the three-dimensional structure of the antigen.
    • This can be used as a vaccine. When the anti-idiotype antibody is injected into a vaccinee, antibodies (anti-anti-idiotype antibodies) are formed that recognize a structure similar to part of the virus and might potentially neutralize the virus.
  • The ultimate goal of Mission Indradhanush is to ensure full immunization with all available vaccines for children up to two years and pregnant women.
  • The Mission is strategically designed to achieving high-quality routine immunization coverage while contributing to strengthening health systems that can be sustained over the years to come.
  • In the last few years, India’s full immunization coverage has increased only by 1% per year.
  • The Mission has been launched to accelerate the process of immunization and achieve full immunization coverage for all children in the country.
  • The Government has identified 201 high focus districts across 28 states in the country that have the highest number of partially immunized and unimmunized children.
  • Mission Indradhanush will target these districts through intensive efforts and special immunization drives to improve the routine immunization coverage in the country.
  • The recommended immunizations for children 0-6 years of age include:
    • • Hepatitis B
    • • Rotavirus
    • • Diphtheria, tetanus, pertussis
    • • Haemophilus influenzae type B
    • • Pneumococcal
    • • Poliovirus
    • • Influenza
    • • Measles, mumps, rubella
    • • Varicella (chickenpox)
    • • Hepatitis A
    • • Meningococcal
  • Four new vaccines are being introduced as part of UIP including Inactivated Polio Vaccine (IPV), Rotavirus vaccine, Measles, Rubella (MR) vaccine, and Adult Japanese Encephalitis (JE) vaccine.
  • With these new vaccines, India’s UIP will provide free vaccines against 12 life-threatening diseases, to 27 million children annually, the largest birth cohort in the world.
  • IPV has been introduced in six states from 30 Nov 2015 to providing double protection against Polio.
  • Inactivated polio vaccine (IPV) Inactivated polio vaccine (IPV) was developed in 1955 by Dr. Jonas Salk. Also called the “Salk vaccine”, IPV consists of inactivated (killed) poliovirus strains of all three poliovirus types. IPV is given by intramuscular injection and needs to be administered by a trained health worker.
  • The inactivated polio vaccine produces antibodies in the blood to all three types of poliovirus. In the event of infection, these antibodies prevent the spread of the virus to the central nervous system and protect against paralysis.

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