A viral infection is an illness caused by a virus, which is a small germ that uses your cells to reproduce. Viruses are composed of genetic material (either DNA or RNA) inside of a protein coating. They are much smaller than bacteria and fungi and can only be seen under a microscope. Viruses invade living cells and use them to multiply, which can kill, damage, or change the infected cells, causing a range of symptoms from mild to severe. Some common viral infections include the common cold, the flu, COVID-19, and HIV. Viruses can infect different parts of the body, including the respiratory system, digestive system, nervous system, and reproductive system. They can be spread through droplets and particles that are breathed out by someone who has the infection, touching surfaces or objects that have the virus on them and then touching your mouth, nose, or eyes, from the pregnant parent to the baby during pregnancy, through contaminated food or water, by being bitten by an infected insect or animal, and through sexual contact.

It can be difficult to know what causes an infection, because viral and bacterial infections can cause similar symptoms. However, bacterial and viral infections are dissimilar in many important respects, most of them due to the organisms structural differences and the way they respond to medications. Antibiotics wont work for viral infections, and misusing antibiotics to treat viral infections contributes to the problem of antibiotic resistance. Therefore, it is important to know if an infection is caused by bacteria or viruses, because the treatments differ.