A virus is a tiny living organism. Viruses are so small they can be seen only with the most powerful microscopes. That's why they're called microorganisms (micro means very small). Other common microorganisms include bacteria Introduction to Bacteremia, Sepsis, and Septic Shock Bacteremia, sepsis, severe sepsis, and septic shock are related: Bacteremia: Bacteria are present in the bloodstream. Bacteremia can result from a serious infection or from something as harmless... read more . Viruses are much smaller than bacteria.
Unlike bacteria, viruses can't reproduce on their own. So when viruses get into your body, they take over certain cells Cells Cells are the basic building blocks of the body. All tissues and organs are made of billions of different cells. Human cells vary in size, but all are quite small. Even the largest, a fertilized... read more 
and use structures in those cells to make more copies of the virus. This usually damages and then kills the cell. However, some viruses can stay inside cells for a long time without killing them.
There are thousands of different viruses. Some viruses infect people. Other viruses infect only animals. Only a few viruses can infect both people and animals.
What is a viral infection?
A viral infection is a sickness caused by a virus.
Viruses can get into your body through breathing air (for example, the virus that causes COVID-19 COVID-19 COVID-19 is a potentially severe viral infection that mostly affects your lungs and airways. It's caused by a coronavirus The virus spreads very easily to other people Some infected people have... read more ), having sex, touching something with viruses on it, or being bitten by a bug such as a mosquito or tick
Viruses usually infect only one type of cell—for example, the virus that causes the common cold infects only cells in your nose, mouth, and throat
When you get a virus, your white blood cells attack it—these cells also remember how to fight it if the same virus gets into your body again
Many viruses make you sick shortly after you get them and then go away
Some viruses don't go away and can make you sick a long time after you get them (for example, HIV Human Immunodeficiency Virus (HIV) Infection and AIDS The human immunodeficiency virus (HIV) is a type of virus called a retrovirus. It causes AIDS (acquired immunodeficiency syndrome), which is life-threatening. HIV is called an immunodeficiency... read more and herpesvirus Herpes Simplex Virus Infections There are 8 different types of herpesvirus. Even though herpes simplex is only one of those types, most people just call it "herpes." Herpes simplex is a viral infection that causes a rash of... read more )
Antibiotics, which treat bacterial infections, can’t treat viral infections
Certain viruses change how your cells work, which can lead to cancer. For example, the hepatitis B and C viruses Overview of Chronic Hepatitis Chronic hepatitis is inflammation of the liver that lasts at least 6 months. Common causes include hepatitis B and C viruses and certain drugs. Most people have no symptoms, but some have vague... read more can lead to liver cancer. HPV Genital Warts HPV is a virus that causes warts. There are many types of HPV. Some types of HPV cause warts on your skin Other types of HPV cause warts on your genitals (genital warts) Some of the types of... read more (human papillomavirus) can lead to cervical cancer.
How can doctors tell if I have a viral infection?
Doctors can tell if you have certain common viral infections based on your symptoms. They may also do:
Blood tests
Cultures (when doctors take a sample from your body and try to grow germs from it in a lab)
How do doctors treat viral infections?
Doctors can’t usually do much to treat many viruses. They’ll mainly suggest medicines to treat your symptoms and help you feel better. For example, if you have a stuffy nose, doctors may tell you to take a decongestant.
For some viruses, doctors can give you an antiviral medicine. Antiviral medicines are used for only a few viruses, including:
Antibiotics are drugs that kill bacteria. Antibiotics do not kill viruses.
How can I prevent viral infections?
Take good care of your health—for example, wash your hands often with soap and water and practice safe sex
Vaccines are shots that teach your immune system Overview of the Immune System The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including: Germs... read more how to fight off certain infections. You usually get vaccines before you're exposed to an infection. But for some viruses, you can get a shot after you're exposed to it. These shots contain antibodies (immunoglobulins) that help fight off the virus. For example, there are immunoglobulin shots for:
