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Nightfalls are not due to HIV or any other form of STDs.
Nightfalls or nocturnal emission (also called wet dreams) involves ejaculation during sleep for a male, and is sometimes considered a type of spontaneous orgasm.
Typically, most men have erections, during the early hours of the day (which is physiological), and orgasms (emission of semen) can frequently occur.
Nocturnal emissions are most common during adolescence and early young adult years. This condition is very common and nothing really to worry about except in very few cases where it is very frequent spread over a long period of time.
The best way to treat this is to masturbate regularly, say once a week or so. This will help in removal of excess of semen, stored within the seminal vesicles and the ejaculatory duct.
Regular exercise and improving physical fitness, as well as Kegel exercises to improve the pelvic muscle strength, will help to control your ejaculation.
The issue will naturally subside when you become sexually active (post marriage), when regular intercourse will result in constant ejaculation of semen, hence solving the issue of nocturnal emissions.
I hope this helps.