What do germs, also called microbes, have to do with humans? Humans (and plants) are a place for germs to live. Think of it this way: humans need certain things to live, like oxygen, food, and water. If you take a human out of this safe, earthly environment and sent him or her to say, the moon, without food, water, and oxygen, he or she would not survive. Germs also need a certain environment in order to survive. And to some germs, the best place to be is inside the human body. Others thrive on our skin, or even just inside our mouths. Germs look for what they need to live.

One important thing to remember is that not all microbes are bad. Many are good and they help our bodies stay in balance. The bad ones, though, can make us sick. The four main types of germs are bacteria, viruses, fungi, and protozoa.

Bacteria are single-celled creatures that live just about everywhere on earth. You name it in the air, in soil, in water, and yes, in and on humans. In these places, their homes, they find nutrients to survive. Bacteria are so small that you cannot see them unless you use a microscope. Just to give you an idea of how small they are, imagine a teaspoon with a BILLION little creatures on it. Those creatures would be bacteria. That means that one bacterium is even smaller than a grain of salt, or the tip of a pin! If you could get a look at different types of bacteria, you would find out that they come in all shapes. Some are shaped like balls, others commas, while others are long and thin like a stick. Some bacteria have longish hairs covering their bodies, which they use like arms to wave around in order to move about. How could bacteria possibly be good? They are many types of good bacteria, including those that live in your intestines and actually help digest your food. One the other hand, there are several types of bad bacteria, including those that cause sore throats or infect a cut.

Viruses need a host to survive. While bacteria can grow and reproduce on their own if they have enough food, viruses need to be INSIDE the cell of a living plant or animal (including humans), or even inside a bacterium! What is the goal of a virus? Once a virus finds the perfect host, the goal is to reproduce and spread. Imagine that a virus makes a home in the cells that make up your blood. Then, it can hitch a ride just about anywhere in the body, and spread itself around. Some viruses will make a home inside a cell and grow and grow until the cell bursts, spreading the virus around to find new "home" cells. Viruses are pretty sneaky because they can mutate (change) quickly to adjust to a new environment.

Fungi are kind of like plants and are made up of many cells. You've heard of mushrooms and yeast. These are types of fungi. The reason that fungi are not called plants is because they cannot produce their own food from soil, water and sun, like green leafy plants (plants can photosynthesize). Instead, fungi live off of animals and other plants. Have you ever seen a tree with fungus growing on it? The fungus survives by living on the tree. So how does a fungus make a home on a human? Fungi love damp warm places, like the underside of a rock or those sweaty cracks between your toes. Of course, the fungus that can make its home on your skin does not sprout out like a mushroom, but it is similar to a mushroom in that all fungi are made up of spores that bud like plants to produce more spores. This is how the fungi can spread. Most fungi are harmless. The kind that can grow on your skin can be treated and looks much like a rash. It probably won't hurt you, but it is really itchy! You can catch a fungus by walking barefoot where it likes to live, like in the school gym locker room.

Protozoa, like bacteria, are extremely small. Of the 20,000 different types of protozoa, most live in water, oceans, lakes, rivers, and ponds. If you were to look at a drop of water from one of these places under a microscope, you would see tiny protozoa whipping their tails around to move. Protozoa actually eat bacteria and they are good because they also eat the waste of other organisms. Some protozoa are parasites, in other words, they live off of other living things, in some cases humans. Malaria for instance is parasitic-protozoa that a person catches from the bite of an infected mosquito. The protozoa get into the blood system. In other cases, if a person drinks contaminated water, protozoa may cause problems in the intestines.