So there’s a risky activity (kid playing in the sewer, me eating Honduran street food, unprotected sex in the third world, etc.)- is it just a SINGLE pathogen particle that can cause the infection, or is a “critical mass” or amount needed? Is it just one cholera bacteria in the food, or are a bunch required for an infection? Is the immune system strong enough for just a few of a certain pathogen, but gets overwhelmed by a large amount?
Depends on the pathogen. Some, like salmonella, you need to have a few hundred or thousand in your body to get ill. Others, like anthrax, you need less than a dozen.
I don’t know of anything which will cause illness with a *single *cell infection, but I wouldn’t put it past some of the buggers!
Remember, when you ingest or inhale something that can make you ill, it will start growing and dividing inside your body, especially if your immune system is weak. So you can ingest 12 salmonella bacteria (not enough to give you symptoms) and within a few days, it can become 20,000 bacteria, which *is *enough to make you sick. For healthy adults, our immune systems should kill off the bacteria before they reach dangerous levels, but you can see why someone who has a weaker immune system, like the very young, very old, cancer patients, diabetics, AIDS patients and transplant recipients are at much greater risk of getting sick after a small exposure.
In theory, a single virus/bacterium can cause an infection and kill you. They grow exponentially, though, so an initial infection of 1,000 puts them 10 generations ahead of a single one. If a generation is 2 hours, then that’s 20 hours advantage by having that many initial infectious agents.
In practice, the initial number is even more important because the immune system takes a little bit of time to gear up to fight the infection. This makes it something of a race where an initial advantage of numbers is important.
While it’s true that the immune system takes a while to ramp up a full and specific response, there are plenty of other more generalized defenses. For one, macrophages are a type of immune cell that hang out near common sites of infection and gobble up everything that’s a bit foreign looking. Then there’s the fact that many pathogens only infect a specific cell type, and really just float around aimlessly until they hit their target. A single pathogen, even one that’s amazing virulent, is even less likely to make it past all of the obstacles our body has.