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Your body is invaded by thousands of microorganisms that will never produce infection. But some microbes can multiply and produce toxic chemicals within the body which damage healthy cells. These microbes are called pathogenic (disease producing) and are recognized by your immune system as harmful "enemies", so when they are detected, an immune response occurs. The immune response produces inflammation, and the "attacks" on the microbes by immune cells produces the pus you see in infections. When the immune response is not sufficient to keep the pathogenic microbes from multiplying, you develop an infection.
The ability to transmit the pathogenic microbe from one person to another means that it is contagious. However, if the immune system of the person you transmit the microbes to can effectively battle the invader to prevent them from multiplying, that individual will not develop an infection. So exposure to the pathogens doesn't necessarily produce infection by the pathogens.
Food poisoning caused by pathogenic microorganisms is an infection by ingested microbes that involves release of toxins by the microbes. Toxins are poisons, and can destroy human tissue cells. So, food poisonings of this type are infections, and also poisonings.
Source(s):
RN