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I understand your concern.
Most sexually transmitted infections have very distinct symptoms, and it can be diagnosed based on history and inspection.
For example, when we consider Herpes infections ( HSV-1/2 ), which is a common STI, Herpes-1 is spread mostly by kissing from an infected partner. The typical symptoms will be cold sores or a cluster of sores in the lips, oral cavity, sore throat, e.t.c
In genital herpes ( HSV-2 ), spread by intercourse, there will be similar cluster of sores in the genital or anal area, followed by a rash which takes around a week or two to resolve. So if there is an exposure, and the patients present with such sores, herpes can be diagnosed without really going for blood tests. Herpes can show up in less than a week if there is contact with an infected partner.
The other common bacterial STDs such as chlamydia and gonorrhoea, again transmitted through unprotected intercourse, presents with symptoms of urethral inflammation and discharge causing a white discharge in the urine and frequent urination. The symptoms are almost immediate and follow a couple of days post the exposure. If such symptoms arise post intercourse with a susceptible partner, treatment is immediately initiated with Ofloxacin/ Doxycycline without really waiting for the results.
HIV is difficult to predict without blood tests, as, in most cases of HIV, there are no symptoms for the first couple of months of acquiring the infection. With the recent 4th-generation tests, it can be identified as early as at the end of 6 weeks.
Candidiasis, a common fungal infection which can also be transmitted sexually, will present with whitish patches on the genitalia and is more of a clinical diagnosis on inspection. This does not require any blood testing either.
Hope this was helpful,
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