Many kinds of infections can happen when bacteria, viruses, fungi or parasites enter parts of the body where they are not supposed to be. Harmful bacteria can cause infections in the skin, gut (GI tract), lungs, heart, blood or brain. Most bacterial infections aren’t serious and can be treated with antibiotics. But if harmful bacteria get deep into the tissues or organs, they can kill you.
Microorganisms, including bacteria, viruses and fungi, live everywhere in the world. They are microscopic and can’t be seen without a microscope. They are so small that they can pass easily from person to person in the air, through food and water or on contaminated objects or surfaces. They reproduce fast and the genetically different organisms that result can adapt to their surroundings quickly. This adaptation enables them to survive and thrive.
Bacteria, for example, produce about 100 new cells every 20 minutes. They have a single circular molecule of DNA that encodes (or programs) their essential genes. They also have accessory, smaller rings of DNA called plasmids that encode for specialized functions like antibiotic resistance.
Viruses, which are made of pieces of genetic information inside a protective shell, spread through direct contact or by a creature that carries them, such as a mosquito or tick. Some diseases, such as influenza and hepatitis B, spread this way. Other diseases, such as Legionnaires’ disease, pertussis (whooping cough) and tuberculosis, spread through contaminated droplets of water or dust in the air.