Health, pathogens and the diseases of bacteria and viruses

When you feel ill

Think of a day when you felt well. You could study, play and sleep without a thought. Now think of a day with a fever. What changed?

The World Health Organization (WHO) says that health is a state of complete physical, mental and social well-being. It is not only the absence of disease. A person with no disease but with no friends, no sleep and no peace of mind is not fully healthy.

Many things decide health. Some are genes, the food we eat, the water we drink, how much we rest and exercise, and the germs around us. A disease may be present from birth, as in a genetic disorder. It may come from an infection. Or it may come from the way we live.

A disease is a change in the body that stops it from working well. A disease that passes from one person to another is infectious, or communicable. Cancer is not infectious, and you will meet it later in this chapter.

Pathogens, parasites and vectors

A pathogen is an organism or a virus that causes a disease. Pathogens come in many kinds: bacteria, viruses, fungi, protozoans (single celled organisms) and helminths (worms). These words are used all the time in this chapter.

• A parasite lives on or inside another organism, the host, and harms it for its own gain.
• A vector is an animal, usually an insect, that carries a pathogen from one host to another. The mosquito is the best known.
• A carrier is a person who has the pathogen but does not feel ill. A carrier can still pass it on.

The picture sorts the diseases of this chapter by the way in which they reach a person. Keep it in mind as you read the next sections.

Typhoid

• Cause: the bacterium Salmonella typhi. Today it is also called Salmonella enterica serovar Typhi.
• How it spreads: by contaminated food and water.
• What it does: the bacteria enter the small intestine, then pass into the blood, and reach other organs.
• Symptoms: a sustained high fever of 39 to 40 °C, weakness, stomach pain, constipation, headache and loss of appetite. In severe cases the wall of the intestine may break, and the patient may die.
• Test: the Widal test.

A person can carry the typhoid bacterium without being ill. Mary Mallon was a cook who carried it and spread typhoid by the food that she prepared. She was nicknamed Typhoid Mary. This is why people who handle food must keep their hands clean.

Pneumonia

• Cause: the bacteria Streptococcus pneumoniae and Haemophilus influenzae.
• Where: the alveoli, the tiny air sacs of the lungs. They fill with fluid, which makes breathing hard.
• Symptoms: fever, chills, cough and headache. In severe cases the lips and fingernails turn grey to bluish.
• How it spreads: by droplets from an infected person, and by sharing glasses and utensils with that person.

The name of Haemophilus influenzae can mislead you. Influenza, or flu, is caused by a virus, not by this bacterium.

The common cold

• Cause: the rhinoviruses, a large group of viruses.
• Where: the nose and the respiratory passage. It does not infect the lungs.
• Symptoms: a blocked nose and nasal discharge, sore throat, hoarse voice, cough, headache and tiredness. These last for 3 to 7 days.
• How it spreads: by droplets from a cough or a sneeze, and by touching things that an infected person has used, such as pens, books, cups, door handles and keyboards.

Dengue and chikungunya

NCERT names these two diseases only when it speaks about mosquitoes. The facts below come from the fact sheets of the WHO.

• Cause: two different viruses. The dengue virus has four types. The chikungunya virus is a different virus.
• Vector: the female Aedes aegypti mosquito, and in some places Aedes albopictus. It bites mainly in the day, and it lays its eggs in clean water that stands in containers, coolers and old tyres.
• Dengue: a high fever, a severe headache, pain behind the eyes, pain in the muscles and joints, and a rash. A few patients get severe dengue, with bleeding and a fall in blood pressure. They need care in a hospital.
• Chikungunya: a fever and severe pain in the joints, often with a headache and a rash. The joint pain may last for weeks or even months.

No medicine kills either virus. Doctors treat the symptoms, and the best protection is to stop mosquito bites and to remove the places where the mosquitoes breed.

Antibiotics

This part is not in the NEET syllabus, but A-Level and IB ask it.

Antibiotics are medicines that kill bacteria or stop them from growing. They have no effect on viruses, so they do not help against the common cold, dengue or chikungunya. If a person takes an antibiotic that is not needed, or stops it too soon, a few bacteria may survive. Those survivors may resist the medicine, and they multiply. This is why a doctor must decide when an antibiotic is needed.

How fast do germs multiply?

A single bacterium divides into two, and each of those divides again. In good conditions some bacteria divide every 20 to 30 minutes. After an hour there may be eight. If the food did not run out, the number after a day would be far too large to count.

You should find that a straight line on the graph means a fixed doubling time. A shorter doubling time gives a steeper line.

Try it in Python

The program counts the bacteria after the doublings.

Practise with new numbers

Common mistakes

• Saying that a cold infects the lungs. The rhinoviruses infect the nose and the respiratory passage.
• Mixing up the cold and pneumonia. Pneumonia is a bacterial infection of the alveoli, and the alveoli fill with fluid.
• Thinking that a pneumonia bacterium causes influenza. Despite its name, Haemophilus influenzae does not cause the flu, which is a virus disease.
• Saying that only a sick person spreads typhoid. A carrier can spread it without any symptom.
• Using an antibiotic against a virus. Antibiotics work on bacteria only.

Remember these

• Health is complete physical, mental and social well-being, as the WHO says.
• Typhoid: Salmonella typhi, food and water, high fever, the Widal test.
• Pneumonia: Streptococcus pneumoniae and Haemophilus influenzae, the alveoli fill with fluid.
• Common cold: rhinoviruses, the nose and the respiratory passage, not the lungs.
• Dengue and chikungunya: viruses carried by the female Aedes mosquito.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How does the WHO define health?

2. Name the pathogens of pneumonia, and say which part of the lung they infect.

3. How does typhoid spread, and which test is used to find it?

4. Why can a cook who is a carrier of typhoid be dangerous, though he feels well?

5. Which mosquito spreads dengue and chikungunya? Are the pathogens bacteria or viruses?

References

• World Health Organization (1946). Constitution of the World Health Organization, preamble.
• World Health Organization (2024). Fact sheets: Dengue and severe dengue; Chikungunya; Typhoid.
• NCERT (2023). Biology, Textbook for Class XII, chapter 7.
• Parker N. et al. (2016). Microbiology, OpenStax, the chapters on respiratory system infections and digestive system infections.