Re: How do pathogens get into our body?

I'm answering this question because it was referred to me. I doubt that your science teacher will regard this means of getting answers as equivalent to looking it up; on your honor, you must tell her or him that you "ordered" this on the Internet.

Our bodies have three surfaces with which we face the world: our skin, the lining of our lungs, and the lining of our digestive tract. Our skin is especially developed to form a constantly renewing barrier — rarely does any pathogen (disease agent) get into our bodies through the unbroken skin. Still, the skin gets broken in various ways fairly often. Abrasions, like skinned knees, may get infected with whatever bacteria were on the skin or on the sidewalk that the knee was skinned on. If your dad shaves, he may cut himself sometimes, and a cut can get infected by bacteria, too. Then there are puncture wounds: if I step on a sharp object, I may need to get a tetanus shot; if I get bitten by a dog, I may need to get rabies shots. Tetanus is caused by a bacterium, whereas rabies is caused by a virus. Bites by insects, especially mosquitoes, may transmit viruses or malaria (a parasitic disease) through the skin, depending on where we are in the world. In developing countries, there are parasites that enter through the skin; these are a particular problem for people who wade in water, planting and tending rice.

Whereas the skin is relatively thick (though it may not always seem so), there is only a one-cell-thick lining between the air we inhale and our blood (in capillaries in the lungs). The total area of the lining of the respiratory tract is enormous. On top of the cells of the lining, healthy lungs, sinuses, etc., are protected by a slimy layer of mucus. Because the mucus is continuously being pushed up and out of the respiratory system by tiny hairlike "cilia" on the lining cells, anything that gets trapped in the mucus is carried out and winds up on our handkerchief, if we use one. If you have been in a dusty place, you can see the dust in the mucus on your handkerchief. If you have an allergy, lots of mucus often comes out, trying to carry the allergen away; but you usually can't see the allergen (say, pollen). Smoking tends to paralyze the cilia so the lungs can't defend themselves. Colds and influenza are caused by viruses that get by the mucus and infect the lining cells. Some bacteria are able to enter by this route, too — one of the most feared these days is the one that causes Legionnaire's disease, but several others can cause serious illness in this way. Tuberculosis infects via the lungs; we don't immunize against tuberculosis in the U.S., as some other countries do, but we do immunize children against whooping cough (bacterial), measles (viral), and other diseases caused by inhaled pathogens. A few parasites are able to enter the respiratory tracts of people swimming in lakes and rivers, but these are very rare.

The third avenue of entry, then, is the digestive tract. Surely, some time in your life, you have gotten sick, and someone told you it was probably food poisoning. Well, maybe it was, although not all illnesses in the digestive tract come from food, and not all foodborne illnesses include the diarrhea and possibly vomiting that we associate with what is usually called food poisoning. The digestive tract works very differently than the respiratory tract — whereas most of the air we inhale goes out again when we exhale, the things we swallow are supposed to go through us. Some gets digested and absorbed, including nutrients to grow on and give us energy; eventually, some of this material goes out via the kidneys (in urine) or the lungs (carbon dioxide from food energy and some of the aromatic components of foods like garlic). The indigestible part of what we eat and drink goes the length of the digestive system and comes out as feces. Because digestion requires enormous amounts of water, the intestines need to reabsorb water efficiently and recycle it via the blood and lymph. When we have diarrhea, much of the water that needs to be recovered gets away, and people who lose too much water this way (for example, with cholera) may be so dehydrated that they die. Bacteria, viruses, and parasites can enter our bodies with food or water and infect via the digestive tract. Some of these pathogens attack the lining of the stomach or intestines and cause digestive illnesses, whereas others invade the whole body and cause symptoms elsewhere. People who get infected with E. coli O157:H7, for example, often have bloody diarrhea; but young children (usually less than 5 years old, but not always) can get "hemolytic uremic syndrome," which makes their kidneys shut down. Eating, and drinking water, are relatively safe in the U.S.; but there are still many pathogens that may infect via the digestive tract on rare occasions. Most young people are not severely affected by these infections; but infants, the elderly, pregnant women, and people whose immunity is impaired by cancer therapy, AIDS, etc., may become very ill. In addition to infections, food poisoning may be caused by real poisons that were either part of the "food" (e.g., poisonous mushrooms) or that were produced by bacteria, molds, or other microbes.

So you see, there are many ways that pathogens can enter our bodies (I didn't even specifically discuss the eyes and ears), and many kinds of pathogens. It is a tribute to our bodies that we stay well most of the time and that most of us live for many years. It is also a tribute to our food and water sanitation systems — in poor countries, where sanitation isn't as good, many children die before their fifth birthday. In the U.S., it is news when a child dies from infection.

Please be sure to tell your teacher where this information came from!