A microorganism, such as a bacterium or protozoan, or a virus, is an agent that causes infection or disease. Pathogens differ from bacteria in that if they enter the body, they can cause disease. The pathogen may be transmitted in a multitude of ways, depending on the pathogen. Skin contact, body fluids, airborne spores, contact with faeces, and touching a surface touched by an infected individual are all ways they can spread.

Immunity refers to the body's capacity to resist pathogen invasion. People are constantly exposed to pathogens, which are foreign disease-causing agents such as bacteria and viruses. Antigens are proteins that are bound to pathogens' surfaces and cause an immune response in the body.

A vaccine is a biological substance that provides active acquired immunity against a specific infectious disease. A vaccine usually contains an agent that looks like a disease-causing microorganism and is produced from weakened or destroyed microbes, their toxins, or one of their surface proteins.