When it comes to infectious diseases, "exposure" means coming into contact with a virus or bacteria. Infection happens when someone is exposed and actually becomes sick from the exposure. Exposure does not always lead to an infection. If the time a person is exposed to the virus is very short, if the amount of virus that enters the body is not in a large enough quantity, or if the body's immune system is able to quickly fight it off, then exposure will be less likely to lead to infection.

Many things have to happen for an exposure to result in an infection, especially the ways in which a person was exposed to the virus. In the case of the virus that causes COVID-19, exposure takes place usually by breathing in the virus through the nose or the mouth, and sometimes the virus enters our bodies through the eyes. People can be "exposed" to different viruses in different ways, such as by eating food with a virus on it, or getting bit by a mosquito or other animal that carries a virus. Again, in the case of COVID-19, exposure typically happens by breathing in the virus through the nose or the mouth.

Other factors that can impact whether an exposure leads to an infection include whether the germ is a virus, a bacteria or a parasite; how strong or "infectious" it is; and the strength of our body defense system (immune system).

For example, you could be exposed to whooping cough (pertussis) by someone in the same room as you, but whether or not you end up being infected depends on several factors. These factors include how close to the person you were, how long you were exposed for, and if you are vaccinated against whooping cough.

We are exposed to viruses, chemicals, sunlight, and many other things every day. In general, exposure suggests that someone has been in an area where they may have come into contact with a foreign substance called a pathogen (a virus, bacteria, or other tiny organism) that can cause a disease. In the case of COVID-19, the pathogen is a respiratory virus.

An infection happens when a pathogen that causes an illness gets into our body (i.e. lungs, skin, eyes, throat, stomach, blood). A virus may enter the body through contact with mucous membranes. These membranes are tissues in the body that line parts of your ear, eyes, nose, throat, digestive tract and other parts of the body that are exposed to air like wounds on the skin. Once the virus enters the body, it enters into cells and is able to spread and cause inflammation. Inflammation causes a person to develop symptoms of infection. Common symptoms of COVID-19 infection include fever, fatigue, cough, and shortness of breath. But scientists are discovering new symptoms associated with COVID-19 every day.

For COVID-19, some people who are exposed to the virus never become infected. The U.S. Centers for Disease Control and Prevention suggests that there is an increased risk of infection for people who are exposed to the virus by being in close contact (within 6 feet or about 2 meters for 15 minutes or longer) with an infected person. The time between exposure to the virus and the appearance of symptoms of infection is currently being studied by many researchers, but we are still learning about this process. For people who become ill with COVID-19, research has shown that symptoms of infection usually begin about five days after someone is exposed to the virus, and 97.5% of people who are exposed to the virus will show symptoms within 11.5 days.