LIWA

Diseases: Cause and Control

ICSE Class 9 · Biology · Chapter 17

What this chapter covers

Key definitions

Disease is a condition in which the normal working of the body or of one of its parts is disturbed, producing signs that can be seen or felt.

Symptoms are the changes in the body that a sick person feels and reports, such as pain, itching, tiredness or loss of appetite.

Infection is the entry, settling and multiplication of a pathogen inside the body of a host.

The incubation period is the time between the entry of a pathogen into the body and the appearance of the first symptoms of the disease.

A carrier is a person who has the pathogen in the body and can pass it on to others, but shows no symptoms of the disease.

Communicable diseases are diseases caused by pathogens that can be passed from an infected person or animal to a healthy person, directly or indirectly.

Non-communicable diseases are diseases that do not spread from one person to another, because they are caused by factors inside the body or by the person's diet, habits or surroundings rather than by pathogens.

An endemic disease is a disease that is always present, at a fairly steady level, among the people of a particular region.

An epidemic disease is a disease that suddenly spreads to a large number of people in a particular area within a short time, well above the usual number of cases.

A pandemic disease is an epidemic that spreads across several countries or continents, affecting a very large number of people worldwide.

A sporadic disease is a disease that occurs only occasionally, as scattered single cases at irregular intervals, with no clear link between one case and the next.

Transmission is the passage of a pathogen from an infected person, animal or object to a healthy person.

Key points

Questions answered

Define a communicable disease and give two examples.
A communicable disease is a disease caused by a pathogen that can pass from an infected person or animal to a healthy person, directly or indirectly. Examples are cholera and tuberculosis.
Distinguish between an epidemic and a pandemic disease.
An epidemic disease spreads suddenly to many people in one area within a short time; a pandemic disease is such an outbreak that spreads across several countries or continents, as COVID-19 did in 2020.
What is meant by a sporadic disease? Give one example.
A sporadic disease occurs only occasionally, as scattered single cases with no link between them. Tetanus is an example.
Why is the housefly called a mechanical carrier, while the female mosquito is a biological vector?
The housefly only carries pathogens on its hairy legs and body from filth to food; the pathogen does not develop inside it. The pathogen carried by a mosquito, such as the malaria parasite, completes part of its life cycle inside the mosquito's body.
Name the vector of (a) malaria, (b) filariasis, (c) sleeping sickness.
(a) The female Anopheles mosquito; (b) the Culex mosquito; (c) the tsetse fly.
Why does a cholera patient need ORS urgently, even before antibiotics?
Cholera kills mainly by dehydration, since the patient loses litres of water and salts in watery stools and vomit. ORS replaces the water and salts at once and keeps the patient alive while the infection is treated.
Give two symptoms of tuberculosis of the lungs and name the vaccine given at birth against it.
A cough lasting more than two weeks, sometimes with blood in the sputum, and evening fever with night sweats and weight loss. The vaccine is BCG.
State three ways in which HIV spreads and two ways in which it does not.
It spreads through unprotected sexual contact with an infected person, transfusion of unscreened infected blood, and shared needles and syringes (also from mother to baby). It does not spread by shaking hands or by sharing food and water.