For a pathogen to make us sick, it must overcome a lot. First it has to enter the body, bypassing natural barriers such as skin, mucus, cilia, and stomach acid. Then it needs to reproduce; some bacteria and parasites can do this virtually anywhere in the body, while viruses and some other pathogens can only do so from within a cell. And all the while, it must parry attacks from the body's immune system.
So while we are constantly inundated by microbes, the number of microbes that enter our bodies is usually too low to get past our defenses. (A tiny enough dose may even serve to remind our immune system of a pathogen's existence, boosting our antibody response to keep us protected against it.) When enough pathogens do manage to breach our defenses and start to replicate, we get sick. Often this is just a numbers game. The more invaders you're fighting off, the more likely you are to feel ill.
How Many Microbes Need to Enter the Body Before We Start to Feel Sick? This varies by pathogen and is known as a microbe's "infectious dose." Usually it takes quite a few, but some microbes require an incredibly small number of organisms to start an infection. Take norovirus for example, the stomach bug notorious for spreading whenever people are in close contact and touch the same surfaces, such as on cruise ships. Its infectious dose can be as small as 18 individual viruses, making it incredibly easy to transmit. It is also very hardy even outside the body, so an infected person who's oozing the virus may leave a large amount of it behind-enough to easily infect others, even several days later.
What About the Concept of "Viral Load"? Is That Related? They're similar ideas, but while infectious dose refers to how many organisms will lead to an infection, viral load is an active measurement of infection: the number of organisms that are replicating within the host. The terminology was first introduced to the general public as part of our understanding of HIV/
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