It could very well happen. In fact you would probably know you fought a HIV infected person when your doctor talk to you about you having a "small" health problem yourself.
So, How do you feel about punching somebody in the face and making him bleed all over the place?
And you grapplers?...You may think: Hey, I'm safe! I use chokes and submissions and I can win with no blood spilled. What about if he bites you to the bone during close contact? If he only licks you in the eyes?...
I believe ring fighters must take tests before entering a competition. It's a good rule but it does not apply to all these gang members ,junkies,prostitutes and their pimps,or even the sick and despaired who want to make others pay for his misfortune!
What do you think?...
And I almost forgot that most of the times,the guy will look O.K. Nobody could tell that he carries a potentionaly mortal disease!