[space_fountain]

I'm not a biologist and this is based on general knowledge and some quick google searches but:

We're quite good at detecting viruses if we really want to. PCR can amplify DNA so much that we can detect even 50 viruses per milliliter of blood. A milliliter seems like actually a lot of blood to get into someone else and your innate immune system is capable of finding and neutralizing small amounts of contagions relatively well even if it's never seen it before. I do suspect this is a statistical impossibility though probably you could somehow get incredibly unlucky, for example in your blood momentarily all the free floating viruses end up in the same bit of blood and that somehow gets into someone else, but I think we can all realize that probability is tiny and in practice I don't think there are any examples of transmission with undetectable levels of HIV.

[et2o]

Well put. We can actually detect well below 50 copies/mL in the laboratory setting as well; we don’t go below this to maintain adequate test specificity and sensitivity in the clinical setting.

[space_fountain]

Not to put you on the spot but from my cellular bio lab I took it felt like PCR should be capable of detecting literally a single example of the targeted bit of DNA in the sample. Is the problem with detecting below 50 that the sample is too easily contaminated or is it that the primers can spontaneously bind to things they aren't supposed to at a high enough rate to invalidate the results?

[jagger27]

> A milliliter seems like actually a lot of blood to get into someone else

1 cm^3 is indeed pretty big. That’s 1,000 little millimetre cubes, and we can detect as few as 50 virions in that space!

I will point out though that 1 ml is low for semen (more like 2.5 ml or more).