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


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


Unit 2
Inside the Body:
How infectious diseases work


SUMMARY
(GRADES 5-7)


Unlike cancer or diabetes, an infectious disease is a disease that can be passed on to others. This is because the 'infectious agents' that cause the disease by attacking the body's cells can move from one environment to another - from dirty water into one's body, for example.

Agents that cause infectious diseases are known as 'pathogens'. The three most important kinds of pathogens are viruses, bacteria and protozoa.

Viruses are the smallest of the three. They are not themselves made of cells. In fact, they need to enter the cells of people and animals to survive and reproduce. Unfortunately, in doing so, many viruses harm our bodies. So while they make their way into our cells only to keep on living, the effect is like an attack on our bodies.

Bacteria and protozoa are different. For one thing they are much larger. Even though we need microscopes to see both bacteria and viruses, their size difference can be as dramatic as the difference in height between you and a twenty story building. Bacteria and protozoa consist of a single cell and so, unlike viruses, do not need to rely on cells from other organisms to survive. Not all bacteria are bad for us. In fact, many bacteria live inside our bodies and perform needed jobs (helping with digestion, for example).

Protozoa

How do these pathogens enter the body and how does the body fight against them?

Pathogens are found everywhere, all around us, and yet people manage to stay healthy most of the time due to the body's own defenses. Sometimes pathogens manage to break through the defenses and cause disease.

The body's first line of defense is made up of the outer, unbroken skin and the 'mucus membranes' lining the body's inner channels, such as the respiratory and digestive tracts which allow us to carry out two essential functions: breathing and eating. Pathogens can get past this first line of defense - through a break in the skin, for example.

White blood cell among red cells Once inside the body, the pathogen can spread killing off our bodies' cells. The body first tries to resist this by calling upon certain white blood cells - the body's second line of defense - which appear in large numbers at the initial site of the infection and swallow up the pathogens.

In some cases, the pathogens will beat the second line of defense as well. Then other, more specialized white blood cells - T cells and B cells - come in as the third line of defense. These cells recognise the foreign agents and cooperate to clean out the infection. B cells produce molecules called antibodies that hook on to pathogens. Antibodies call out to the other specialized cells, like T cells, to come in and destroy the infecting pathogen. The body's ability to defend itself in this way is called immunity.

In some cases, we may not have the necessary antibodies, in which case there are vaccines. In a sense, vaccines are medecine's contribution to the strength of our bodies' immune systems.

A vaccine is simply a preparation containing a very weak form of the infectious agent or pathogen. Once the vaccine is injected, the body produces antibodies against that particular pathogen. Because the pathogens are weak, the body kills them off quickly. But the antibodies stay around for a period of time to guard against the same pathogen.

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