Infectious Diseases and Immunity: Quick Revision

A brief note on Infectious Diseases and Immunity. Read it before a test. Try each question at the end before you open its answer.

Infectious agents

• Five groups: viruses (cold, influenza, dengue, AIDS), bacteria (typhoid, cholera, tuberculosis, anthrax), fungi (skin infections), protozoa (malaria, kala-azar) and worms (elephantiasis).
• Viruses live and multiply only inside host cells.

How an infection spreads

• Air: cold, influenza, tuberculosis.
• Water and food: cholera, typhoid fever, jaundice.
• Contact and blood: AIDS.
• Mother to child: through the placenta or at birth.
• Vectors: the female Anopheles mosquito (malaria), Aedes (dengue), the housefly.
• The organ where the microbe settles decides the signs: lungs (tuberculosis), liver (jaundice), brain (encephalitis).

Treatment and prevention

• Treat the symptoms to ease the patient. Treat the cause to remove the microbe.
• Antibiotics block steps that bacteria need. They do not work on viruses.
• Take every dose the doctor ordered. If you stop early, resistant bacteria survive.
• General prevention: clean water, food and surroundings, no stagnant water, fresh air.
• Specific prevention: immunisation.

Vaccines and public health

• The immune system kills microbes and remembers them.
• A vaccine is a harmless form of a microbe. It trains the immune system without causing the disease.
• Edward Jenner used cowpox against smallpox. Vaccines exist for tetanus, diphtheria, whooping cough, measles and polio.
• When most people are vaccinated, the unvaccinated are safer too: herd protection.
• HIV attacks the white blood cells. It spreads by sexual contact, infected blood and from mother to child. It does not spread by touch.

A worked example

A school has $500$ pupils, and $425$ are vaccinated. What percentage is vaccinated, and how many are not?

1. Fraction. $\dfrac{425}{500}$.
2. Percentage. $\dfrac{425}{500} \times 100 = 85$ %.
3. Not vaccinated. $500 - 425 = 75$ pupils.

Practice with new numbers

A program to try

The program finds the vaccine cover in a school.

Mistakes to avoid

• Antibiotics do not act on viruses. They are for bacteria.
• A vector is only a carrier. The microbe comes from an ill person.
• One vaccine protects against one disease.
• HIV does not spread by touch, hugs or shared food.
• Do not stop an antibiotic early.

Check yourself

1. Name the five groups of infectious agents.

2. What is a vector? Give one example.

3. Why should every dose of an antibiotic be taken?

4. How does a vaccine protect us?

5. In a school of $600$ pupils, $480$ are vaccinated. What percentage is vaccinated?