What is an infectious disease?
An infectious disease is caused by a pathogen. A pathogen is a very small living thing (or particle) that harms the body. Infectious diseases can pass from one person to another, so they are also called communicable diseases.
A non-communicable disease does not spread from person to person. Diabetes, most heart disease and most cancers are examples. They come from genes, age and lifestyle.
The four main types of pathogens
- Bacteria: single cells. They multiply fast and some make poisons (toxins). Examples: cholera, typhoid, tuberculosis (TB). Many can be treated with antibiotics.
- Viruses: much smaller than bacteria. They live and copy themselves only inside our cells, then burst out. Examples: flu, measles, common cold, HIV. Antibiotics do not kill viruses.
- Fungi: examples: ringworm and athlete's foot on the skin.
- Protists: single cells with a nucleus. Example: malaria, carried by mosquitoes.
Some worms (like roundworm) also cause disease.
How pathogens spread
- Air: tiny droplets from coughs and sneezes (flu, TB, measles).
- Water and food: dirty water or badly cooked food (cholera, typhoid, food poisoning).
- Direct contact: touching a person, their blood or body fluids, or shared objects (ringworm, HIV through blood).
- Vectors: animals that carry the germ from one person to another. Mosquitoes carry malaria and dengue. Warmer weather lets mosquitoes live in new places, so these diseases can spread further.
How the body defends itself
First line: skin is a tough wall. The nose has hairs and sticky mucus. Stomach acid kills many germs in food. Tears contain chemicals that kill bacteria. Useful bacteria living on the skin and in the gut also crowd out harmful ones, so our body is like a small ecosystem.
Second line: white blood cells. Some swallow (engulf) germs. Others make antibodies that stick to one kind of germ, and antitoxins that block poisons. After an illness, memory cells remember the germ, so we fight it faster next time. This is immunity.
Vaccines, medicines and prevention
A vaccine contains a dead, weak or harmless part of a pathogen. The body makes antibodies and memory cells without getting really ill. If the real germ comes later, the body kills it fast.
When most people are vaccinated, the germ cannot find new people to infect. This protects even those who cannot take the vaccine. It is called herd immunity.
Medicines: antibiotics kill bacteria only. Always finish the full course; misuse makes bacteria resistant. Painkillers ease symptoms but do not kill germs.
Simple prevention: wash hands with soap, drink clean water, cook food well, cover coughs, keep sick people at home, use mosquito nets and remove standing water. Before travelling, check which vaccines and precautions are needed for that region.
Key formulas and definitions
- Pathogen: a microbe that causes disease
- Communicable = can pass from person to person
- Vector: an animal that carries a pathogen (e.g. mosquito)
- Antibody: protein made by white blood cells that sticks to one germ
- Vaccine: harmless form of a germ that trains immunity
- Herd immunity: enough people immune that the germ cannot spread
- Antibiotic: medicine that kills bacteria, not viruses
Worked examples
1. Rahul has a cold caused by a virus. His friend says, "Take antibiotics." Is this right?
No. A cold is caused by a virus. Antibiotics only kill bacteria. Rest, fluids and time help; a doctor decides on any medicine.
2. A village has many malaria cases after the rains. Suggest two ways to cut the spread.
Malaria is carried by mosquitoes (vector). 1) Remove standing water where mosquitoes breed. 2) Sleep under mosquito nets and use repellent. Blocking the vector blocks the disease.
3. In a class of 20, 16 children are vaccinated against measles. One child brings measles. Why do very few fall ill?
The germ needs a non-immune person to jump to. 16 of 20 are immune, so most contacts lead nowhere. The chain breaks early. This is herd immunity, and it also protects the 4 unvaccinated children.
4. Classify: cholera, flu, ringworm, malaria, diabetes.
Cholera: bacterium (water). Flu: virus (air). Ringworm: fungus (contact). Malaria: protist (mosquito). Diabetes: non-communicable, no pathogen.
Common mistakes
- Thinking antibiotics cure viral diseases like cold or flu. They only work on bacteria.
- Saying all microbes are harmful. Most bacteria are harmless, and many are helpful (in the gut, in curd).
- Thinking a vaccine is a medicine for someone already sick. It works before infection, by training immunity.
- Calling the mosquito the cause of malaria. The cause is a protist; the mosquito only carries it (vector).