Infection occurs when disease-causing microorganisms enter your body and multiply. They can affect any part of your body and cause a wide range of symptoms. Infections are caused by bacteria, viruses, fungi and protozoa, including the parasitic worms that cause malaria and other diseases.
Some infections are symbiotic, meaning that they help your body in some way. For example, some types of colonizing bacteria prevent the adhesion and invasion of other pathogenic bacteria in the wound, while others can kill off bad bacteria that invade a healthy cell. Some infections are contagious and spread to other people, such as pertussis (whooping cough), tuberculosis, strep throat and some bacterial sexually transmitted infections. Others are not contagious, such as infections of the skin or gastrointestinal tract. Bacterial infections may be spread through direct contact with an infected person, or indirectly through contaminated objects or surfaces. Infections that are spread by bugs, such as mosquitoes, ticks and fleas, are called vector-borne infections. These include Rocky Mountain spotted fever, Lyme disease and shigellosis.
All multicellular organisms are colonized to some extent by non-pathogenic microorganisms, which form a mutual or commensal relationship with their host. Some of these microorganisms, such as bacteria and fungi, may become pathogenic under certain conditions, including a compromised immune system. This is why it is important to maintain a strong, well-functioning immune system through healthy eating, exercise, avoiding unnecessary stress and limiting antibiotic use.