Infection occurs when germs (usually bacteria, but also viruses and fungi) get inside the body and multiply. Infections can be serious, even life threatening.
Bacteria, which make up the majority of infections, are single-celled microorganisms that have learned how to do many things that other organisms can’t: they stick to surfaces, make paralyzing poisons and other toxins, evade or suppress our immune systems, and resist drugs. They’re responsible for illnesses ranging from impetigo and boils to tuberculosis, pneumonia, meningitis and strep throat.
Viruses that cause infections include hepatitis C and HIV/AIDS. Some types of infections are caused by bacteria that invade and infect the bloodstream, causing septicemia or other severe complications; these include fever, weakness, sweating and low blood pressure. Others, such as the ones affecting the heart, lungs and urinary tract, can lead to high fever, chest pain, shortness of breath, night sweats, difficulty breathing and a build-up of pus in the lungs or blood vessels (pneumonia).
Some infections are spread when people in close contact share saliva or mucus (like phlegm or snot) through direct physical contact. Other diseases, such as the flu and some sexually transmitted infections like gonorrhea and chlamydia, are spread when we breathe in droplets or dust that contain bacteria or other organisms. We can also be infected when we touch contaminated objects or food and then rub our mouth, nose or eyes, or by getting ticks, fleas, mosquitoes or other creatures (called vectors) infected with pathogens.