Brief Answer:
Yes it is possible to get infected without semen discharge or blood

Detailed Answer:
Hi,
Thanks for writing in.

Yes you can get infected with disease causing bacteria and viruses if you insert your penis in to another persons anus if there is no semen discharge and blood.

If there is no semen or blood still there is a small amount of secretions and body fluid in the anus which can transmit germs and infect you.

Please use condoms for protection if you prefer to insert your penis in another persons anus.

Hope your query is answered.
Please do write back if you have any doubts.

Regards,

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