This section tells what happens when a person is infected by a pathogen, and why some people get sick while others dont.
Frist, there are several factors necessary for a communicable disease, which are: an intermediary host, a means of transmission, and a host.
The intermediary host is wherever the pathogen, whether it is a bacteria, virus, or whatever, stays until it finds a suitable host that is not already infected. This could be a plant, a pet, a person, or dirt.
And as for means of transmission, that's just the way they are spread. This can happen many ways. One is close contact with an infected person. This means if someone who has a cold sneezes on you, there's a good chance that you'll get a cold. Another means of transmission is direct contact with the pathogen. A good example of this is last years outbreak of Cryptospiridium. The bacteria was in the water, and when people drank the water, they got sick.
Still another way to spread a pathogen is contact with animals. An example of this is rabies. If you're bitten by a rabid animal, you could get rabies. Another example would be getting Lyme disease from a deer tick. One more means of transmission is contact with objects.
But this section dealt with more than just the ways communicable diseases are spread. It also talks about the body's defenses against disease-causing pathogens. The body has 3 major defenses: mechanical, chemical, and biological defenses. A kind of mechanical defense is the layer of dead skin cells. A chemical defense is the digestive juices in your stomach. Being highly acidic, this kills some, but not all of the germs in the food you eat. An example of biological defenses are special white blood cells called neutrophils. These cells surround and eat any invading bacteria. There's also some other white blood cells called monocytes, which do the same thing as neutrophils.
Another biological defense is fever. When the body temperature goes up, some bacteria cannot function. This can kill the bacteria, or it can weaken them so the white blood cells can kill them.
The body's immune system attacks pathogens in two ways: by white blood cells digesting bacteria and by production of antibodies. Antibodies are proteins that are found in the blood, and they neutralize or kill pathogens.
Part of the immune system is the lymphatic system. The lymphatic system plays an important role in fighting infection. Lymph carries more white blood cells, such as granulocytes and lymphocytes, B cells and T cells. B cells produce the antibodies found in the blood. T cells directly attack pathogens, surrounding and digesting them.
Antibodies, as I said before, are proteins that attack pathogens. They work in various ways, like by dissolving bacteria, or clumping pathogens together, so the white blood cells can do their work. Antibodies are highly specialized and work only on certain pathogens.
The main points you're supposed to get out of this section are: