Our body is amazing at keeping pathogens at bay. The moment our immune system detects a foreign body, it springs into action to combat the unknown assailant. The body will react by sending neutrophils and lymphocytes to kill the pathogen. However, there are cases when our immune system has been compromised and it's not strong enough to fight off the infection, and that's when we get sick. However, normally, we have enough defenses to prevent that from happening, unless the microorganisms that entered our system are able to reach what is called their "infectious dose", which is the number of microbes it takes to make us sick.
Tara C. Smith elucidates more about the infectious dose on Quanta.
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