If there was a small, cheap, simple device or test that anyone could obtain, which at any time could be used to immediately indicate if anyone was positive or negative for chlamydia, gonorrhea, herpes, HIV, HPV, syphilis, hepatitis, trich, chancroid, LGV, NGU, scabies, CMV, mono, etc., what would happen?
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This would only make a large change if this test was available for self administration (somewhat like a pregnancy test kit), and allowed the user to test their potential partners before a sexual encounter.
If this was the case, then sexual mores and behaviours would change. If a person refuse to submit to a test, they would be dropped as a potential partner. Soon, the signal for an encounter might be "will you take my test?" (perhaps followed by a coy smile). As time passes, it will become considered strange for sexually active people not to have a testing kit on their person at all times, or saying you don't have a kit might be a polite (or not so polite) way to turn a person down. The institution of marriage might come under more stress, if it becomes common practice to demand a spouse submit to the test (this might be considered a not so subtle accusation of infidelity).
Unfortunately, there is a large enough reservoir of irresponsible or uncaring people out there who would neither carry the kit or demand testing (or submit to testing) but would still be sexually active. This means that while large scale social pressure to use the test would bring the rates of STD's down considerably, there will still be reservoirs of infection out there. The other thing this test might mask it the evolution and spread of new diseases that use sexual encounters to spread to new hosts, since the STD kit would be optimized to detect currently known diseases.
Firstly: healthcare professionals worldwide would get hold of this device and reverse engineer it for any other number of disease identification purposes. I feel this isn't really the answer you're after though, so:
Not a grand lot more than already happens.
The main issue with the efficacy of STD tests isn't how simple or cheap they are. It's that often people don't want to consider (or it just doesn't cross their mind due to ignorance or overconfidence) that they might need one. These tests would make it one hell of a lot easier for the safety conscious to check for STD's, but when it comes to transmission those aren't the people you need to worry about.
This device would be incredibly useful if mandatory testing were enforced (and in fact you'd pretty much need it to be able to even consider mandatory testing) but the kind of future where HIV is eradicated by enforced testing and social vilification of those infected is not a future I think most people want to live in.
One possibility would be if this device were a near field communication implant that instantly tells you when it detects an STD. That way people would slowly all start getting the implant just for peace of mind, as it would require no more thought after implantation. That then leads to the revolution in infection control that I think you're after.
Also: give serious thought to using such a device for all other infectious diseases. WHO would love it.
I heard a story on a news magazine a while ago -- can't find a transcript so I can't give the exact quotes -- but they said that a study had found that almost 30% of people who were infected with an STD got it from a partner who had no visible signs of the disease. The panelists talked about how scary this was, how hard it was to protect yourself, etc. Then one panelist said that he thought that the really scary part was that over 70% of people who were infected with an STD got it from a partner who DID show visible signs of the disease. And then they apparently went ahead and had sex anyway.
So even if there was a 100% reliable test, apparently lots of people wouldn't bother to use it, or would ignore the results. The more difficult it is to use, the more expensive, the more trouble it is to get, etc, the fewer would use it.
And let's face it, how many people in a moment of passion are going to say, "Wait, before you finish taking off your clothes, let me run down to the drug store and get an STD test kit. I'll be right back. Wait right there."
They're actually fairly quick and accurate at the moment. Cost depends on whether you're in a country that scalps you for medical care or one that provides it for free.
In the "West", you'd end up with a massively reduced rate of transmission of STIs among intelligent educated responsible people, who would require a test before any sexual encounter, and a moderately reduced rate among the reckless and uneducated. I expect the US would charge people $150 a time for the test kit, it would be flagged on your medical record and future employers would be informed that you once had sex. The rest of the civilised world would get it for free from the local doctor or pay a token fee at a pharmacy. Similar to access to contraceptive pills today.
You'd get major campaigns for testing and treatment in African countries who would end up with reduced transmission rates across the population, though not total reduction due to the difficulties of teaching people that having sex with a virgin doesn't cure HIV (this makes me cry).
The more religiously oppressive regimes would ban it immediately for encouraging infidelity and probably stone anyone caught in possession of a kit.
The second sexual revolution.
For purposes of a more compelling answer than "What would happen? Well, basically whatever is happening now, because you've described something only slightly better than today's level of technology", I'm going to assume the following important things:
- "immediately indicate" means that the test will successfully pick up an infection with effectively zero incubation period (say, within a matter of hours after intercourse)
- 100% sensitivity and specificity (all infections are identified, with no false positives)
- as with existing cheap & instant HIV and pregnancy tests, the comprehensive test is available for home use (can be self-administered, as Thucydides pointed out)
Conditions two & three are definitely important, but I feel that number one is key. This, combined with the comprehensiveness of the test, would provide ironclad assurance that a potential sexual partner was not a health risk.
The Achilles' heel of the current generation of STI tests is that they are based on antibody response -- which is variable, and takes too long. This is a nuisance for the responsible, and the root of the sexual health threat posed by those who are not. With an instant test, waiting for incubation periods to pass, and indeed, waiting for test results at all, would become a thing of the past.
The stigmatization of premarital sex took a heavy hit when contraception and abortion became widely available. The stigma of multiple sexual partners being a health safety concern will likewise come under fire, as the threshold of effort and self-control required to be cautious with one's personal sexual health would be drastically lowered.
It would become socially expected to test oneself in the presence of a new sexual partner prior to intercourse, as a gesture of openness and respect. Refusing would be like not buckling your seat belt in a car -- what possible excuse could you have? It is to be hoped that it could even be packaged directly with traditional forms of contraception.
Additionally, we have two things now widely available that the first sexual revolution did not: social networking and location-aware communication devices. It would be short work to adapt already-existing dating apps for more overt purposes. This creates the potential for unintended and possibly unsettling consequences, like the network-wide "blackballing" of STI-positive individuals.
Via the Internet this is already available however of course there are disclaimers. And what would happen if this was available as you suggest I'm sure the instructions at the bottom in small print would read in case of any positive test result please refer immediately to a licensed health care provider for further assistance. Blah blah blah in a nut shell we not responsible you are so i think that puts you back to square one. However on a positive note there may be a smal decline in the spread of stds since id like to believe there are caring responsible people still in this world who may have been embarrassed to see a doctor or could not afford