An infection occurs when another organism enters your body and causes disease. The organisms that cause infections are very diverse and can include things like viruses, bacteria, fungi, and parasites. Viruses are very tiny infectious organisms. They’re even smaller than bacteria. On the most basic level, a virus is composed of a piece of genetic material that’s surrounded by a protein shell. Some viruses may have an additional envelope or other features on their surface. Viruses are parasitic and require a host cell in which to carry out their life cycle. Once the virus has entered the host cell, it’s able to use cellular components to reproduce. New viruses are released from the host cell, a process that’ll sometimes cause the host cell to die. Bacteria are single-celled microorganisms. They’re very diverse, coming in a variety of different shapes and sizes. Bacteria can be found in all sorts of environments, including soil, bodies of water, and in or on our bodies. Some can survive extreme temperatures or even radiation exposureTrusted Source. Although there are a great many bacteria in and on our bodies, these bacteria often don’t cause disease. In fact, the bacteria in our digestive tract can help us digest our food. Fungi are another diverse group of organisms that can include things like yeasts and molds. They can be found throughout the environment, including in the soil, indoors in moist areas like bathrooms, and on or in our bodies. Sometimes fungi are so small that you can’t see them with the naked eye. Other times, you’re able to see them, such as when you notice mold on your bathroom tile.