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WHAT ARE BACTERIA AND VIRUSES?

The most common infectious diseases are caused by

bacteria and viruses. Bacteria are single-celled

organisms that live outside our cells. They multiply in

body fluids and wounds. Many are killed with drugs

called antibiotics. Viruses are microscopic particles

with their own genetic material. They live in our cells

and take them over, but cannot survive without them.

PNEUMONIA BACTERIA 3

Pneumonia is a lung disease which

can be caused by bacteria or a virus.

This electron micrograph shows

pneumonia-causing bacteria (shown in pink) on

the tiny hairs (green) that line the lungs’ airways.

Pneumonia can affect anyone, but people who are sick

and so have a weakened defence system are particularly vulnerable.

WHAT ARE GENETIC DISEASES?

Genetic diseases, such as cystic fibrosis and

muscular dystrophy, occur when a person inherits

a particular gene or combination of genes that is

harmful. Sometimes the harmful genes are passed

on unknowingly by unaffected parents; in other cases

the gene changes at conception to become abnormal.

The study and treatment of disease is called medicine. Modern

medicine aims to prevent and cure disease where possible. Some

diseases, such as diabetes, are incurable, but medical treatment

can still relieve symptoms and improve a patient’s quality of life.

WHAT IS CANCER?

There are many different types of cancer, but what

they all have in common is that cells start to divide

uncontrollably, forming a tumour. The tumour may

then prevent part of the body from working normally.

This is sometimes caused by agents called

carcinogens, such as sunlight or tobacco smoke.

WHY DO SOME PEOPLE SUFFER FROM ALLERGIES?

The body has sophisticated defences for attacking

germs, called the immune system, but sometimes the

system attacks the wrong targets – people have an

allergic reaction when the immune system is triggered

by harmless substances like pollen and dust.

HOW ARE DISEASES TREATED?

The body can cope with most illnesses itself, without

medical help. We recover from a sore throat, for

example. For major problems, a range of treatments

can be used, such as drugs, surgery, or radiotherapy

(the killing of harmful cells using radiation).

CAN DISEASES BE PREVENTED?

Many infectious diseases are prevented by

vaccination, which protects the body from specific

germs. Screening programmes can detect the early

stages of diseases such as cancer. A healthy lifestyle

can also help prevent conditions such as heart disease.

Disease

4 HIV

The virus that causes AIDS

(acquired immune deficiency

syndrome) is called HIV (human

immunodeficiency virus). The

virus invades cells of the immune

system, making it more difficult

for the body to defend itself from

the virus and other diseases.

MEDICINE

BLOCKED ARTERY 3

X-rays are one of the many

medical imaging techniques that

doctors use to investigate and

diagnose disease. This X-ray

reveals a blockage in a major

artery (yellow) in a person’s leg.

This image clearly shows

the surface of the virus. The

glycoprotein complex that sticks

out enables the virus to attach

to and fuse with target cells,

such as white blood cells, and

then initiate an ongoing

and infectious cycle.

Blood flow

in this femoral artery

has stopped just

above the knee

Femoral artery

with normal blood flow

Femur

FIND OUT MORE. Biotechnology 208 • Body 128–129 • Cells 73 • Micro-organisms 85 • Robots 194

Anything that damages the body or how it works can

be called a disease. Some diseases are caused by

microscopic organisms (germs), others by injury,

poisons, or the body itself. Nearly all diseases can

be treated by modern. MEDICINE to some extent.

disease