sometimes! i don’t know specifically for HIV how many particles are needed to infect. but i know for norovirus it’s something wild like only 10 viral particles need to enter the body to cause infection. this is the same for shigella, about 10ish bacterial cells. comparing to salmonella which is like thousands of cells (different sources say different numbers but on average about 10^5 or 10^6 cells: 100,000 or 1,000,000). in the case of these 3 pathogens it’s different because they’re gastrointestinal & i think the number of cells has to do with survival in stomach acid (part of your immune system, called the innate part) but my point for saying all of this is that yes, at some point for all pathogens your immune system will kill all of the particles but the issue is when there are too many particles at once for it to kill. 1 particle of a pathogen, yeah you got this. 2 particles? probably. 100 particles? not so sure. because another factor that comes into play is how good the pathogen is at evading the immune system. for the example i used above, shigella isn’t as affected by stomach acid as salmonella is, so shigella can “evade” by surviving and entering your intestine which is why only small numbers are needed to infect. on the other hand salmonella isn’t so good at evading the immune system/the stomach acid, so you need like a hundred thousand cells because sooo many of them are going to be killed by the stomach acid that with a higher number of cells, there are higher odds that a few cells will survive and make it through to your intestine. with lower numbers of cells entering the stomach, the odds are higher that the stomach acid will destroy all the cells and none will make it to your intestine, therefore you do not get sick bc your immune system/stomach acid worked!

i hope this helps explain the general concept even though i could not focus on HIV specifically. feel free to ask me any more questions (even if you think they’re lame - they’re not!! i love explaining and talking about micro stuff!!) and if there is something specific you really want to know i can help you find good sources that explain it

EDIT: i also forgot to add because i haven’t seen it really spelled out in the few comments i’ve read. but for pathogens living on surfaces, if the surface is “clean” it’s generally harder for a bug to survive a long time (“long time” being different for each pathogen) but if a surface is “dirty” it is generally easier. so by dirty, i mean if there is actual dirt or biological material, like dried blood or body fluids. so to help you picture it, at home let’s say you want to disinfect your stove top but there is dried food all crusted on. that is a “dirty” surface because of the dry food. and in order to disinfect, you need to first clean the surface to remove all crusty food & once you do that it is a “clean” surface. then you can disinfect properly. the reason you can’t disinfect straight away on a dirty surface is that whatever Stuff is on there can provide some sort of environment or nutrient etc to pathogens and it’s hard to penetrate the Stuff with disinfectant, so you first need to clean to remove the Stuff which removes a safe spot for the pathogen, and then when you disinfect there is no Stuff to penetrate and you can ensure disinfectant remains on the surface for the given contact time

Not a lame question at all! It’s astute. Viruses differ in the amount of virions necessary to cause infection because the immune system sometimes does have the ability to fight a particular virus before you get symptoms.

In the case of HIV, HIV positive people on Prep or other drug therapy AND with a very very low titer have been shown to not transmit the virus to intimate partners.

Measles requires a very low amount of virus to cause illness while a fairly large dose of rhinovirus can be handled by the immune system without causing symptoms.

Viruses and our immune systems are fascinating and I could go on all day! Lol

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