Infection occurs when viruses, bacteria, fungi or protozoa enter your body and multiply. They may cause disease in some people, such as a cold, the flu, measles or a bacterial skin infection (psoriasis). Infection can also be caused by parasites like hookworms or worms that live inside your intestines.
Most infections come from germs that spread from person to person. Viruses, which are tiny pieces of genetic material wrapped in a protein shell, can be spread through close contact, such as shaking hands with an infected person or letting your fingers touch a cut that hasn’t been cleaned. Viruses can also be spread through body fluids, such as blood or urine. Some infections, including hepatitis B and human immunodeficiency virus (HIV), are spread by needlestick injuries or sharing used injection equipment.
Bacteria are single-celled microorganisms that can be seen under a microscope and come in three shapes: spherical (coccus), rodlike (bacillus) or curved (vibrio, spirillum or spirochete). They typically carry one circle of DNA, called the main genome, but may have accessory small rings of DNA, known as plasmids, that encode specialized functions such as antibiotic resistance.
Most multidrug resistant organisms, or MDR organisms, grow best in moist environments and tend to accumulate on wet surfaces and in biofilms. They are also more likely to spread when a person infected with an MDR organism coughs or sneezes, creating droplets that contain the germ and land on others. Infections can also spread when people handle an infected animal or its waste without wearing disposable gloves or washing their hands.