Lesson 2 of 13 · 7 min
Bacterial and viral diseases
NCERT §7.1
In the fever ward a girl who ate at a roadside stall has run a steady 39.5°C for five days, and the boy in the next bed only has a runny nose. Dr Leela says one of them needs a blood test and the other just needs rest.
The lesson in notes
In short
Pathogens are disease-causing organisms: bacteria, viruses, fungi, protozoans, helminths and others. Most parasites are pathogens, because they harm the host they live in or on.
A pathogen must adapt to its host. One that enters through the gut has to survive the low pH of the stomach and resist the digestive enzymes.
Typhoid is caused by the bacterium Salmonella typhi. It enters the small intestine in contaminated food and water and spreads to other organs through the blood.
Typhoid signs: a high fever that stays at 39° to 40°C, with weakness, pain in the stomach, constipation, headache and poor appetite; severe cases can end in intestinal perforation and death. The Widal test confirms it.
Mary Mallon ('Typhoid Mary') was a cook and a typhoid carrier who kept spreading the disease for years through the food she prepared.
Pneumonia is caused by Streptococcus pneumoniae and Haemophilus influenzae. It infects the alveoli, which fill with fluid and make breathing very hard. The patient has fever, chills, cough and headache, and when it is severe, the lips and finger nails go grey to bluish.
Pneumonia spreads by inhaling droplets or aerosols from an infected person, or by sharing their glasses and utensils. Dysentery, plague and diphtheria are other bacterial diseases.
The common cold is caused by rhino viruses, which attack the nose and the respiratory passage but spare the lungs. Nasal congestion and discharge, sore throat, hoarseness, cough, headache and tiredness usually last 3-7 days.
Cold droplets from coughs and sneezes are inhaled directly or picked up from contaminated objects such as pens, books, cups, doorknobs and a computer keyboard or mouse.