I'm wondering HIV viability outside the body of an infected person, for example in a knife that contains blood of an infected person, with N virus cells. (N could be for example, 1000). Some virus might live more than others, then I'm asking the half-life of it.
The general response to that question is not long, the HIV is a weak virus and that once exposed to air, it can survive for maybe a few minutes at best.
BUT Under specific conditions, HIV can survive for a far longer period of time, sometimes for hours or even days if the right temperatures, pH balance, light exposure, and humidity are achieved. It is a very difficult set of conditions but is nevertheless possible.
- In certain conditions, the virus can survive outside the body for several weeks.
- Survival depends on which body fluid it is in, volume of the body fluid, concentration of the virus within it, temperature, acidity, and exposure to sunlight and humidity.
- HIV transmission has not been reported as a consequence of contact with spillages of blood, semen or other body fluids.
If you've come into contact with some blood or other body fluid that you think might contain HIV, it's understandable to have some concern about the possibility of HIV transmission. But you can rest assured that there haven't been any cases of HIV transmission through casual contact with blood or semen that has left behind on a surface. There haven't even been any cases after people have come across discarded syringes or needles.
This is partly because it's extremely unusual for this situation to involve any opportunity for an infected body fluid to enter the person's bloodstream -- it does not reach a mucous membrane (such as the vagina or rectum) or an open wound.
So in practical terms, there's little reason to worry about contact with body fluids that have already been outside a person's body for some minutes.
There isn't a simple, straightforward answer to the question of how long HIV survives outside the body. In certain, specific circumstances it may survive more than a few minutes. But it generally does not remain infectious and certainly does not pose a threat to people's health. The conditions that a body fluid is exposed to greatly affect survival. Air dries out the fluid, which contains the virus, greatly reducing viral amounts. On the other hand, in the enclosed space inside a used syringe (with limited exposure to air) the virus can survive some time -- this explains why re-using needles and syringes is risky.