A virus is a tiny infectious agent that can make people, animals, and even plants sick. Viruses are so small that you need a powerful microscope to see them: millions of viruses could fit on the head of a pin. Unlike bacteria, which are living cells that can survive on their own, viruses can only reproduce by invading the cells of living organisms and hijacking their machinery.

When a virus enters your body, it attaches to your cells and injects its genetic instructions inside. The infected cell becomes a virus factory, creating thousands of copies before bursting and spreading more viruses to other cells. Your immune system fights back by producing antibodies and killing infected cells, which is why you feel sick: fever, coughing, and fatigue are signs your body is battling the invader.

Common viral illnesses include colds, flu, chickenpox, and measles. Antibiotics don't work against viruses because they target bacteria, but vaccines can train your immune system to recognize and defeat specific viruses before they make you sick. Scientists have developed vaccines for dangerous viruses like polio, smallpox, and rabies, saving millions of lives.

In computing, a computer virus is a program that spreads by copying itself into other programs, behaving similarly to biological viruses by spreading from computer to computer and potentially causing damage.