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Diseases Due to Pathogens

  1. Diseases caused by viruses
  • Common cold, caused by rhino virus, is the commonest infectious disease which spreads through moisture droplets during sneezing and coughing.
  • Influenza or flu is an acute respiratory tract infection caused by influenza virus, Myxovirus influenzae.
  • Mumps is caused by Paramyxovirus. This disease is characterised by a very painful swelling in the neck below the ears, where it affects the parotid salivary glands. It spreads through handkerchief, crockery and utensils used by the patient.
  • Poliomyelitis or polio is primarily a disease of children. In severe cases, it causes paralysis of the limbs. In mild cases, it may even go undetected. It is caused by a small virus having a single-stranded RNA as the genetic material. The disease spreads through faecal-oral route.
  • Rabies/hydrophobia is an acute, highly fatal viral disease of the central nervous system. A type of rhabdovirus causes rabies. It is a disease of warm blooded animals such as dogs, cats, monkeys, rabbits etc. It is transmitted to man usually by bites or licks of rabid animals.
  • Hepatitis or jaundice infects the liver. Hence, it is a liver disease. Hepatitis is caused by any of half a dozen viruses; hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV), hepatitis E or G virus. Hepatitis B is a DNA virus and the rest are RNA viruses. The disease is characterised by yellow colouration of the skin, eyes and urine. It spreads through unhygienic food and contaminated water.
  • AIDS (acquired immuno-deficiency syndrome) is caused due to virus HIV (human immunodeficiency virus) which weakens the human body immunity or self-defence mechanism. It is a RNA virus. The victim becomes vulnerable to a host of life-threatening infectious, neurological disorders or unusual malignancies. It spreads through sexual contact, blood transfusion from infected person and infected syringes.
  1. Diseases caused by moulds and fungi: Diseases such as ringworm and athlete’s foot are caused by moulds and fungi.
  • Ringworm is a disease of skin, hair or nails, marked by ring-shaped patches on the skin and spread through contaminated clothes and articles.
  • Athlete’s foot: The fungi attacks the skin of foot particularly between the toes.
  1. Diseases caused by protozoa
    Malaria is a disease caused by protozoans of the genus Plasmodium. This parasite grows and multiplies in the liver and the RBC’s of man, and the gut of the female Anopheles mosquito, which is the carrier or vector. The protozoans enter our bloodstream when a female mosquito carrying them bites us.
    Malaria is characterised by periodic attacks of shivering followed by high fever. Headache and muscular pains are also common.

Life Cycle of Plasmodium That Causes Malaria

Amoebic dysentery
 is caused by the Entamoeba histolytica. It is transmitted by food, water and flies. Symptoms include weight loss, anaemia, diarrhoea, fever, fatigue, destruction of lining of large intestine leading to blood and mucus in stools and liver infection.

Treatment and/or prevention include sanitation, control of flies and antibiotics such as metronidazole.

Life Cycle of Entamoeba histolytica That Causes Amoebic Dysentery

  1. Diseases caused by bacteria
  • Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis. The mode of transmission is through air, dust and sputum of the infected person. Symptoms include persistent cough, afternoon fever, blood mucus, loss of weight and fatigue, chest pain and breathlessness.
  • Cholera is a diarrhoeal disease caused by Vibrio cholerae. The mode of transmission is through contaminated water and food. It also spreads by flies. It is characterised by the sudden onset of profuse, watery diarrhoea followed by vomiting, rapid dehydration, muscular pain and suppression of urine.
  • Tetanus or lock jaw is caused by Clostridium tetani. The mode of transmission is through cuts or wounds in the skin. Symptoms of the disease include locking of the jaw due to muscle spasm.
  • Syphilis is a sexually transmitted bacterial disease. It is caused by the microorganism Treponema pallidum.
  • Typhoid is a bacterial disease caused by Salmonella typhi. It is transmitted directly through soiled hands contaminated with faeces or urine of patients or carries. It is also transmitted by the ingestion of contaminated water, milk, food or through house flies. It is characterised by continuous fever for 3–4 weeks, weakness, slow pulse etc. Diarrhoea may also occur. Fever with headache rises to maximum in the afternoon.
    Other bacterial diseases include diphtheria, whooping cough, pneumonia, leprosy, gonorrhoea etc.
  1. Diseases caused by parasitic worms
  • Ascariasis is caused by common round worm (Ascaris).
  • Taeniasis is caused by tapeworms (Taenia solium).

Filariasis or elephantiasis is caused by Wuchereria bancrofti.

Life Cycle of Taenia solium




Life Cycle of Filarial Worm

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