The central job of our immune system is to protect and defend our body against pathogens, or microscopic organisms. When pathogens invade our body, they can cause sickness and disease. Three major groups of pathogens are fungi, bacteria, and viruses.
Pathogenic fungi are eukaryotic microorganisms and are usually part of the plant kingdom which include molds and yeasts. Fungi produce spores and grow well in warm, dark, moist areas living on dead tissue. These conditions occur often between your toes or under your nails after long days covered with warm, sweaty socks. As a result, a common fungal infection called athlete's foot occurs. Other fungal infections are yeast infections and ringworm. Most fungal infections are eradicated by the immune system without notice, but when the immune system is compromised or suppressed, as with AIDS, fungal infections can take hold. Fungal infections are much more difficult to treat because of their eukaryotic cell structure. Unlike bacteria, fungi do not respond well to antibiotics and need to be treated with highly toxic chemicals.
Bacteria, commonly referred to as, "germs", are single-celled, prokaryotes. Prokaryotes are microorganisms with no nucleus. They are living organisms able to move, eat, and reproduce. Bacteria are able to reproduce with incredible speed. In fact, one bacterium can become millions in just a few hours.
There are many kinds of bacteria. Some are "good" bacteria, which aid in maintaining health and some bacteria are "bad" and can make you very sick. In actuality, we can live in harmony with thousands of good bacteria on our skin and in our intestines. For example, the good bacteria that live in the intestines help to break down food. However, harmful bacteria, called bacterial pathogens, can make you very sick. One example of a bacterial pathogen that can enter your body is the streptococcus bacteria. This bacterium is able to live on surfaces and can enter your body through the mouth. Upon entering the mouth, this bacterium directly invades the cells in your throat and kills them, which causes strep throat. Other bacterial pathogens can destroy cells indirectly by producing toxins when they replicate. Lyme disease is another example of an illness caused by bacterial pathogen which enters the body through the bite of a deer tick. Other harmful bacteria can enter the body through your airways. Examples of these bacterial pathogens are tuberculosis, meningitis, and chicken pox. Tuberculosis lives in the water droplets in the air and can cause infection when it is breathed into the lung tissue. Still other bacteria gain entry through the water and food we eat and drink. E coli and cholera are two examples of these deadly pathogens. There are many good bacteria, but our immune system is in a continuous battle against a host of bacterial pathogens as they try to gain entry into our body.
A virus is a tiny piece of DNA covered with protein. Although bacteria and viruses are both pathogens, they are very different. Viruses are much smaller than bacteria and can pass through small places that would filter out bacterial pathogens. Unlike bacteria, viruses are not considered to be alive. As a result, viruses are not able to reproduce like bacteria. Instead, a virus must take over a living cell and use its parts to replicate. The way a virus takes over a cell is by attaching itself to the cell membrane and then injecting its DNA into the cell. Next, the viral DNA uses the cell's parts to copy itself. In this way, the viral DNA hijacks the living cell's DNA and uses it to clone itself. The virus then continues to replicate in the host cell, like a factory, until the cell membrane can no longer contain this expanding volume. At this point, the infected cell explodes. This bursting event releases about 10,000
viral clones, freeing them from the cell to find and infect new host cells. In this way, the viral pathogens are able to invade, multiply, and then simultaneously destroy their host cell while releasing thousands of new clones to invade new host cells. Carl Zimmer suggests that there are over 10 million times more viruses than there are stars in the universe . Imagine the spectrum of illness and diseases these viral pathogens cause. Some examples of a viral illnesses cause by these pathogens are influenza, rabies, herpes, hepatitis, and AIDs. Since, these viral infections are so challenging for the immune system, they are important foci of bioengineers.