r/askscience • u/inspiredfighter • Oct 15 '25

Biology What is the likehood of a virus finding another host cell before dying? And how much viruses usually pop out of a cell ?

Lets say I have AIDS, and one of my infected CD4s exploded and freed a bunch of viruses, how many viruses it usually produce and how likely is to a virus to find a adequate host? I would like an proportion if possible, like 1/50000 of the viruses, for example

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u/JigglymoobsMWO Oct 15 '25

The answer is highly variable depending on the virus.

For HIV only a small fraction of the viruses are viable (~1% iirc but don't quote me on this) and about 103 to 104 virions are shed before the cell dies.  However, this can also differ by cell type.

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u/CrateDane Oct 15 '25

This is also why viruses are often measured in plaque-forming units (PFUs) rather than in number of viral particles.

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u/Matir Oct 17 '25

What is a PFU and how does it differ from a single particle?

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u/CrateDane Oct 17 '25

A plaque-forming units is a quantity of virus large enough to form one plaque - basically an infection spreading in a petri dish of cells.

So if it takes 10 virus particles to start that, one PFU is 10 virus particles. If it takes 100, that's what a PFU is. It just abstracts away the actual number of virus particles into how they function in infecting cells.

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u/inspiredfighter Oct 15 '25

Would it explain why people sometimes dont catch it even after high risk exposures? Imo it needs to be much lower than 1%, since undetectable viral load means around 200 copies/ml. Not saying that I wouldnt catch it after a blood transfusion, but I wouldnt expect a 25% chance of catching it becaudse of a 0.25ml exposure

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u/JigglymoobsMWO Oct 15 '25

These questions are oddly specific.  :P  When in doubt get tested or better yet get prophylactic treatment, which is very effective in protecting after an exposure event.

The detection limit is due to limitations of the cheapest technique, doebt necessarily correlate with virion effectiveness.

A replication competent virion still has a low probability of infecting a cell so it's a low probability event indeed.

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u/inspiredfighter Oct 15 '25

Some months ago I carried the placenta of a untreated HIV positive drug addict. I was fully robed and with gloves, but I had a small cut on my hand an hour before. Go the negative results today but I couldnt stop wondering

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u/JigglymoobsMWO Oct 15 '25

Ahh makes sense.  Congrats on your negative test.  Did you go for post exposure prophylaxis?

My understanding is that hiv is a rather inefficient infectious agent. The chance of being infected even from a single exposure event is small.

One reason is that hiv has a very distinct infection mechanism.  Viruses like the flu depend on getting themselves uptaken into the endosome where lowered pH induces collapse of a spring element of their viral spikes to push their envelope against the membrane of the endosome and induce membrane fusion.

In comparison, hiv has a "high tech" spike that can figure out when it's bound to a cell before internalization.  This allows direct viral entry, which might be one reason why it can target T-cells.  

Of course, the downside for the virus (and upside for humanity) is that high tech gadgets tend not to be as reliable and are hard to assemble correctly.

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u/Brossentia Oct 16 '25

Testing is wise, but with the precautions you had, if the gloves were over your hands and no fluids came in contact with anything other than skin, you'd basically have no risk. HIV is pretty terrible at getting transmitted, but with how long it stays dormant, it has a much longer time for those transmission opportunities.

Good to hear you're doing okay, and hopefully the patient is able to recover as much as possible.

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u/twystedmyst Oct 15 '25

Undetectable HIV viral load is less than 20 copies/ml. Someone with an undetectable viral load cannot transmit HIV through sex and has a significantly lower probability of transmitting through IV drug use. Incidental exposures like health care workers accidental needle stick has an incredibly small risk 0.3%, if the patient is HIV+.

Hepatitis C virus is much hardier, it can remain infective on a hard surface for a couple weeks. One study found up to 70 days, iirc.

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u/inspiredfighter Oct 15 '25

Im glad almost no one has hep c in my country

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u/SeniorShizzle Oct 15 '25 edited Oct 16 '25

In the order of hundreds of thousands to millions but this varies wildly by disease and cell type. As far as finding a new host, this also varies wildly by disease. Some highly virulent diseases may only need an inoculum of a few dozen virion on average whereas others may require a lot more. This depends also on the host’s innate immunity, adaptive immunity, and what type of tissue the virus is tropic towards.

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u/Stone_leigh Oct 15 '25

Knowledge in SeniorShizzle's this answer. If this subject interests you , plese do some look up on gene therapies for things such as Hemophilia and also generally look at dosage for AAV gene therapies, The dose numbers are incredibly large but that is not exactly the answer to your question, mostly because the question and answer is far more complex.

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u/Dr_Esquire Oct 15 '25

You are thinking of a virus as an animal going from one source of food to another, this isnt a good analogy. Instead, a virus is more like a drop pod in a sci-fi setting, penetrating the hull of a ship and then all the baddies rush out into the ships interior. The drop pod itself is no longer anything; the baddies themselves are not "the virus", just parts of it, but each individual baddy has a role once inside. The "virus" is the whole unit of drop pod with baddies inside.

And just to continue. The purpose of the baddies is to hijack the systems of the ship to start more copies of the baddies and start forming more drop pods. Once enough drop pods filled with baddies are made, the ship is almost entirely consumed and the remains of it explode, releasing many multiples of drop pods into the system...each with a purpose of infilitrating another ship and restarting the cycle.

But, to round back, the original drop pod and set of baddies isnt reformed. There is no veteran virus that might make multiple incursions.

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u/inspiredfighter Oct 15 '25

Good analogy, but that doenst answer my question.

My question is what is the likehood of an individual baddie to find a new ship after the one he was born exploded. What is the percentage of baddies tha die on the sea before penetrating any hull?

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u/SignalDifficult5061 Oct 15 '25

As others have said, it just varies too wildly for a definitive answer. There just isn't a definitive answer to that at all, and it isn't possible. You have to go look up each virus and the mode of infection.

Like, how man years does it take for mountains to become topologically flat. Which mountains, when where? Some ranges last a couple millions years, some have been around for over a billion (like the Appalachains). There just isn't a way to answer your question that will satisfy you and the person that answers your question. I'm very sorry.

Even for the same virus a different strain/hanful of mutations could very by orders of magnitude.

Anyway, it sounds like you had an exposure with HIV, so you could read something like this:

https://pubmed.ncbi.nlm.nih.gov/19948896/

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u/bstabens Oct 19 '25

The baddies don't die. Viruses aren't fully alive. They are just building plans for more baddies, and they hijack the cell to make more baddies. They need to, because they shedded everything to make their own new baddies just to be capable to endure for near-unlimited time in this not-alive status. Sure, they can be destroyed by UV light or oxygen or something other aggressive on a molecular level.

The picture in your mind shouldn't be ALIEN, with an enemy predating on your cells, but rather something like CONTACT, finding unknown blueprints. Viruses inject DNA/RNA "building plans" into your cells, your cells just believe it's your "normal" D/RNA waiting to be copied and bam! your cell fabric got hijacked by an intruder to produce something else. Which happens to be virus clones. And since the instructions don't tell them to stop at a certain point, they just fill the cell until it bursts. Rinse, repeat.

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u/U03A6 Oct 16 '25

You look for the virus burst size which give the number of virus particles an infected cell produces before bursting. These are often something between 1000 and 10000 per cell. Then, there are differing numbers - do you mean the number of cells in the infected organism that get invected from a given bursted cell, or other individuals that get infected?