First I wouldn't say anything unless I was 100% sure....Go get tested...and if in fact you do have an STI/STD....
Both Parties need to be informed. How you tell the person depends on how well you know them, but I would never approach it in an accusatory manor.
Just let them know that you went for a check-up and found out you had one and they should get tested as well.

Both providers and hobbyists have an obligation to protect one another, though not all practice this. You would be surprised to know the percentage of people who know they have an STD that still hobby or provide. I would not even attempt to guess, but being 1 in 4 people have a form of Herpes and over 90 percent of the population has some strain of HPV, it's a safe bet that the numbers are pretty high within our community.