Infection is when disease-causing germs (bacteria, viruses, fungi and parasites) grow inside the body. They may affect one part of the body, like the skin or lungs, or spread throughout the whole body. Some infections, like strep throat, are contagious and can be caught by other people. Others, such as bloodstream infections or infections of the heart valves, are life threatening.
Bacteria are usually the cause of infections that affect the skin, eyes or genitals. They can also invade the bloodstream and spread to other parts of the body, causing symptoms such as high fever, sweating and weakness. Fungi (spore-forming organisms that range from bread mold to deadly ringworm) and parasites (like pinworms, hookworms and tape worms) are also infectious.
The most common way to get infected by bacteria is through direct contact with the infection-causing microorganism, such as touching contaminated skin or mucous membranes, including the mouth, nose and eyes. Other ways to get infected include getting bitten by a bug that carries disease-causing germs (like mosquitoes, fleas or ticks) or by ingesting them through contaminated water or food. Diseases that spread this way, such as campylobacter, E. coli, Salmonella and some sexually transmitted infections like gonorrhea and chlamydia, are called vector-borne.
Some bacterial infections, such as impetigo and boils, are contagious, but most of them are not. Bacterial infections that develop in the lungs or heart can be spread from person to person through droplets of liquid or dust that contain the pathogens, such as during an asthma attack or during a sneeze.