I know that HIV is primaraly transferred by sexual encounter, iv drug use, mother to baby and in medical settings (transfusion, donor transplant, stick injury, etc)

However, there is this 5% of people who state they do not know how they got infected. This promted me to believe that a few things that happen to me recently can be a route of transmission. 

In particular: 1) Someone handed me a piece of paper with a smudge of fresh blood on it. I did not notice it at first and touched the paper, my phone and headphones which i put into my ears. After noticing I disinfected my hands and there were no cuts/wounds on my hands as well, but would that be considered a risk of transmission (considering only a few seconds passed before i took the paper into my hands)

2) Frozen yogurt. I was eating frozen yogurt the other day, it tasted funny. I looked at it and it had what looked like blood on it. I had a little cut on my lip. Would that be a risk or HIV cannot survive in yogurt even if during a manufactoring process some of it got into the mix accidently (far fetched, I realise, but still wondering)

3) I visited a dentist one month ago. It was late in the evening (i was the last patient). The clinic looked clean, etc, but I wondered if there can be some contamination between patients. I tested one month later with a home test and it came back negative. However, just yesterday, when I visited the clinic again, something flew into my eye before the procedure started. It was dust, probably, but i am not sure. Different scenarious come into my head like it could had been dried blood or something like that. Although I know it is more far fetched than the yogurt thing. 

So would you think any of the above can be routes of transmission. What about those people who say they do not know how they got HIV...In your practive and learned opinion they just do not want admit to it etc or HIV is much easier to catch than I realise? Thank you

However, you're missing a key point. In the US, the unknown transmission figure of 5% comes from officially reported figures, i.e. as reported by infected patients and then summarized in statistics compiled by health departments and then summarized nationally by CDC. In the large majority of such cases, no further investigation is done. However, study after study has shown that when the initial "unknowns" are investigated in more detail, the traditional, expected risk factors emerge.

The large majority of people with HIV have had multiple potential exposures and really don't know where and from whom they were infected. Others knowingly lie about it, i.e. saving face by not admitting homosexuality, injection drug use, etc. Others aren't intentionally untruthful or simply don't recall an obviously risky event; and many are the regular partners of deeply closeted persons at risk, especially women whose husbands/partners have sex with men. When more intensive interviews are undertaken, and all partners are also interviewed and tested, traditional sexual or drug use histories almost always come to light.

In addition, many of the sorts of exposures you describe are simply not plausible from a scientific standpoint. Everything we know about the biology of HIV and the amount and kinds of exposures required for transmission argue against risk from any of the events you have described. I think you don't understand that HIV transmission requires that large amounts of virus -- not just a single virus -- come into direct contact with susceptible cells that are usually not exposued from the sorts of things you mention. For example, vaginal sex, with infected semen deposited inside the vagina, carries an average transmission risk of 1 infection for every 1,000 exposure. What then can it be from more superficial contact? Similarly, swallowing HIV (if there really were some blood in the yogurt, for example) carries very little risk. Among babies nursed by infected mothers, after swallowing a few ounced of HIV contaminated milk every day, after 6 months only 15% of babies become infected. So how much risk can there be from a single swallowing exposure of a drop or two of blood?

So I would encourage you to not worry about the sorts of events you have described above. There is simply no credible evidence of any possible risk from them. Have only safe sex and ideally maintain committed relationships with one partner at a time, and avoid drug use with shared injection equipment -- and you'll never have to worry about HIV.

I hope this ibnformation is helpful. Let me know if anything isn't clear.

HHH, MD