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Fighting Disease:
Health At The End Of The Millennium
Another Wired Curriculum from The United Nations CyberSchoolBus

1 Introduction
2 How They Work
3 How They Spread
4 Poverty and Disease
5 Prevention
6 Immunization

Unit 3
Transmission of Infectious Diseases

(GRADES 5-7)

We are told to cover our mouths when sneezing or coughing. We are told not to share a glass with someone who's sick. We are told to wash our hands before coming to the table and not to eat food that has been dropped on the floor. Why?

The reason is simple: by following these rules we are preventing disease-carrying microbes from spreading. Microbes are small and can be carried from place to place in different ways - one of which is through 'droplets' such as those produced when coughing or sneezing; another way can be through contaminated food or water.

At the same time, there is no need for exaggerated alarm. Not all diseases are as easily transmitted as, say, influenza. And not all diseases are transmitted in the same way.

Here are some of the ways in which major infectious diseases get transmitted to people.

People Person-to-person

If not careful, people can transmit certain types of infections to each other.

People can pass on infectious diseases to others through droplets of moisture produced when coughing or sneezing. These droplets can carry certain microbes. Influenza is a virus transmitted by droplets. A more dangerous one is tuberculosis, one of the re-emerging diseases.

Direct contact and bodily fluids

There other infectious diseases that can be passed on from one person to another but not simply through droplets. Polio and leprosy are passed on through direct contact. Some diseases, such as HIV/AIDS, can be transmitted sexually. Others, such as hepatitis, are passed on through blood.

Water Food and water

Both food and water, if contaminated, can transmit infectious diseases to humans.

Waterborne diseases are a serious problem in poorer areas of the world where lack of safe water contributes to spreading infectious diseases such as cholera, dysentery and typhoid. The biggest danger comes from diarrhoeal diseases which are caused by contaminated water and food and kill up to 3 million people, mostly children, every year.


Like water, contaminated food can carry bacteria that cause diarrhoeal diseases. A number of other bacteria are transmitted by food only. Most significant among these are salmonellae, campylobacter and Escherichia coli. These are present all over the world. Estimates for food borne diseases in the United States range from 6.5 million to 80 million cases a year.

Mosquito Insects

Many insects carry viruses and protozoans which they transmit to humans (these insects are called 'vectors'). Of these, the mosquito is the greatest menace.

Mosquitos can spread an astonishing range of diseases: malaria, dengue and yellow fever are three examples. These three together are responsible for several million deaths a year.

Other insects

Other insects carry diseases too: household bugs cause Chagas disease, fleas carry the plague, and tse-tse flies spread sleeping sickness.

When diseases spread very fast to a large population they are called epidemics. Preventing transmission is possibly the most important way to combat disease - as the old saying goes, an ounce of prevention is worth a pound of cure.

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