3

As far as I've heard, you can only get an STI if it is transmitted to you by someone who is infected with one.

Then the following questions arise:

  • How can anyone ever get an STI if it has to be transmitted in the first place?
  • How does the originator of the STI become infected? It would have to be transmitted by someone with an STI.

I don't understand how you can become infected if that requires you to have sexual relations with someone who's already infected.

I'm stuck in a vicious loop on this one.

  • 4
    There's nothing special about STDs. You could ask the same question about the common cold. – Carey Gregory May 22 '19 at 14:21
  • Some STDs remain for life and have serious symptoms - a cold doesn't. – MrStack May 22 '19 at 16:57
  • What is an "originator?" Do you mean historically in the ancestry of humanity?? I recommend reading on the basics of infectious disease. – DoctorWhom May 22 '19 at 16:58
  • 2
    Incidentally, regarding STI vs STD, that subject has been covered thoroughly in a previous question. – Carey Gregory May 23 '19 at 0:52
  • 2
    @MrStack You keep going back and forth about whether you are talking about someone getting infected today or something like patient zero (and I tried to answer both). My answer refers to the study of patient zero for HIV where the answer seems pretty clearly that the first human HIV infection was from chimpanzee blood. As CareyGregory pointed out, "how would patient zero get infected" is a question that applies to every type of infection regardless of how it spreads after. If you clarify where your confusion lies it would help us clarify the answer. – Bryan Krause May 23 '19 at 14:58
6

How are STIs different from other pathogens?

STIs are infections of pathogens that either live in the genital area and/or exist in body fluids and also usually don't get transmitted in simpler ways such as airborne particles or contaminated surfaces.

Sexual contact is simply one of the few ways that humans exchange those fluids or have skin contact between the genital areas of two individuals and the by far most common way for the disease to be transmitted.

For example, Staphylococcus aureus bacteria live very happily in the genital regions and could certainly be transmitted sexually, but we don't think of Staph as a sexually transmitted disease because it's found in/on other places too and is also transmitted easily without skin to skin contact.

Where do STIs originate from in the human population?

Bacteria and viruses that today are thought of as sexually transmitted are simply those that have specialized to infecting and spreading in the genital area. Given that there are relatives of those pathogens that have other patterns of infection, and given how evolution operates more broadly speaking, it is straightforward to say that sexually transmitted infections are caused by pathogens that evolved from similar species with other patterns of infection/transmission or other host species.

HIV is a special case in that it's a relatively new STI for humans and its history has been closely studied (see Sharp & Hahn, 2011). Viruses similar to HIV seem to have been in monkeys for a long time, and at some point were transmitted to chimpanzees, probably due to blood contact between chimpanzees and monkeys they were eating, and most likely spread to humans from chimpanzees in the same fashion. Once a human was infected in this fashion, the primary way they spread the infection to other humans was sexually.

Where do STIs originate from in a given person or sexually active couple

Given two people who are not infected with STIs, they cannot spread STIs between them through sexual contact: STIs do not originate spontaneously.

However, someone who currently does not have an STI can in the future become someone who does have an STI and then spread infection to their partner, primarily by sexual contact with some third party who is infected.

It is possible for sexually transmitted pathogens to also be transmitted in other ways; for example, for several STIs transmission is possible during childbirth, herpes viruses can be transmitted by mouth contact, HIV is transmitted by blood contact such as contaminated blood supplies or reused needles in situations of poor medical practice or illicit drug use (or in the patient zero case described above for HIV, through blood contact of another host).

Someone infected in any of these ways can then spread the infection to a sexual partner.


Sharp, P. M., & Hahn, B. H. (2011). Origins of HIV and the AIDS pandemic. Cold Spring Harbor perspectives in medicine, 1(1), a006841. DOI

  • Does this imply that two people who both don't have STD's or STI's will never get them by having sexual relations with each other? – MrStack May 22 '19 at 18:16
  • 1
    @MrStack That is correct. – Carey Gregory May 22 '19 at 18:22
  • 3
    @MrStack Though it is possible for sexually transmitted pathogens to also be transmitted in other ways; for example, for several STIs transmission is possible during childbirth, herpes viruses can be transmitted by mouth contact, HIV is transmitted by blood contact such as contaminated blood supplies or reused needles in situations of poor medical practice or illicit drug use, etc. – Bryan Krause May 22 '19 at 19:18
  • 3
    @CareyGregory Interesting question; I suppose for me I've always thought of "sexually transmitted" in terms of a category label for pathogens that are "sexually transmissible" as a primary way of spread, rather than describing the specific way that any individual is exposed, but I have no hard evidence on how many people think of it that way. – Bryan Krause May 23 '19 at 2:26
  • 1
    I tend to see it like @BryanKrause with emphasis on transmissible - - interesting point! – DoctorWhom May 23 '19 at 5:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.