Infection is when microscopic living beings, such as bacteria, viruses, fungi, and parasites, get into your body and multiply. These microorganisms (also known as germs) are everywhere. Most of the time, they don’t cause illness in people and some help with important functions like breaking down food or removing waste from the body. But if the wrong microorganism gets into the body under certain conditions, it can make you very sick.
Many infectious diseases spread from one person to another through direct contact, such as touching a cut or scrape that’s infected with bacteria or viruses. Others spread through indirect contact, such as breathing in the droplets that spray out when a person coughs or sneezes. This is how respiratory infections, such as the common cold and influenza, spread. And sexually transmitted infections, such as chlamydia and gonorrhea, are spread through all kinds of sex (oral, anal, and vaginal).
Many infectious diseases also happen when pathogenic organisms adapt to changes in the environment or their hosts. These adaptations can include new ways to invade the body (e.g., antibiotic resistance) or new genetic changes that increase the virulence of a parasite, allowing it to infect more victims. New diagnostic tests, such as metagenomic sequencing, are being developed to identify an infection based on the presence of the organism’s DNA, rather than by detecting antibodies. This approach is particularly useful in cases where the host is immunocompromised, as it may be difficult to detect antibodies against a particular pathogen.
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