Getting a hand job is seen as a very low risk activity.
There are a number of infections that can spread from skin to skin contact, such as scabies. It is more likely that scabies is passed from full body contact rather than a hand job e.g. someone sleeping with you.
Sexually transmitted infections such as chlamydia theoretically could spread if the person has infectious fluids on their hand e.g. they touched their vagina and then gave you a hand job. This is not a common way to transmit chlamydia.
There was discussion in regard to the risk of transferring genital warts. The research around this is very grey. Theoretically it may be possible to spread warts from your genitals to your hand and then to someone else’s genital.
Again I could not find any research that looked at the effectiveness of washing with different products to prevent STI/HIV transmission. Soap and water is great for getting rid of many bacteria and viruses on your hands. So it makes sense to wash your hands before giving a hand job. Be careful not to over wash with products that may irritate the skin.
If you wanted to reduce the risk further you could wear a glove e.g. the type that health care professionals use.
In conclusion I just wanted to stress that this is a low risk activity.
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