1. Hot tubs. It is what you do in the hot tub, not the hot tub that sometimes leads to STD transmission. The tub and tub water itself will not spread STD.
2. No, the foreskin is thinner skin than most skin and is sometimes irritated to the point of scraping and bleeding during vigorous sex. This is not an STD however. if the irritation occurs immediately after sex or in the 24 hours following sex, it is not an STD. EWH
Thanks for such detailed answer. Only 2 more questions:
1) Can I get one of these STD´s in a hot tube in those gay saunas? I mean, by the water itself?
2) With my behavior (as described above) I think I got a kind of micosys in my penis (foreskin - I´m not circumcision). My foreskin has some scratch and bleed sometimes. Is it a type of STD´s? Can I transmit it to someone?
Welcome to our Forum. I'll try to provide some perspective but in the end, you are correct, there is no act involving other people which is, at least theoretical, entirely safe. Of course the same can be said about walking outside (you could be struck by lightning), shaking hands (you could get the flu and die), or simply sitting inside a building (it could collapse on you). As we go through life I suggest that you try to achieve some balance which allows you to interact with others while preserving a tolerable degree of safety. I'll try to give some broad answers to your rather general question however, I will not try to give you details about specific STDs (including HIV), or sex acts. if you want more specific information you'll need to ask another question or go to a textbook.
As one considers sexual interactions with other people, the most important determinant of risk is who your other partners are. Do they have lots of other partners?, do they take other risks with their health?, do they act to make sure they do not have an infection by taking advice about regular check -ups, etc? Thus who your partner is, how well you know then, and how well they take care of themselves is a major determinant of STD risk. Anonymous partners who go to venues where having casual, anonymous sex is part of their reason for existence is clearly a place where you are relatively likely to encounter an infected partner. Interestingly however, in such situations, the majority of people are not infected. Further, please remember that it is a biological fact that only a small proportion of sexual exposures to an infected partner lead to transmission - most do not.
Clearly, if your partner has an STD, penetrative sex is the most likely to result in transfer of any sort of infection, from HIV to gonorrhea or chlamydia and on to lesion diseases such as syphilis, herpes or HPV. And, among sorts of penetrative sex, it is clear that being the receptive partner is the act which is most likely to result in acquisition of infection while being the active partner is oral sex is probably the least likely to result in transmission of infection. Condom use acts to reduce the likelihood of infection to a major degree.
With non-penetrative sex (mutual masturbation, frottage) some of the most common STDs are virtually never transmitted (gonorrhea, chlamydia, trich) while other lesion diseases (syphilis, herpes, HPV) are occasionally transmitted but substantially less often than in penetrative encounters. As a result such encounters tend to be considered safe as transmission of infection is, on average, quite rare. Further, as it turns out, the skin of non-mucosal surfaces is less likely to become infected with any STD than the genital skin (which is thinner and more vulnerable than say the skin of the leg or a hand) which is turn is less vulnerable than the skin which makes up mucosal surfaces.
Putting all of this together, with judicious choice of partners, caution in sex acts and use of protection, while not "perfectly safe" it certainly is possible to have a healthy sex life. I hope all of this is helpful. EWH