Ok, I have to jump in here. According to the CDC, there has never been a case of HIV contracted from a public tattoo shop. As of last year the only possible case was in a prison, and it was intentionally inflicted. It happened to use a tattoo crude tattoo machine, it could as well been anything a prisoner could have sharpened and used within a few minutes of applying the infected blood. The HIV virus is temperature specific, and dies very shortly after being exposed to air. Most of the diseases mentioned in previous posts and also prison related, or done at home by unqualified people, or scratchers, that purchase kits from unethical supply houses that do not include sterilizers or any advice on how not to spread disease. Whoever makes these statistics does not bother to tell people that.
Granulomas or granular lesions are usually associated with poor quality or home made inks. Red reaction occurs less and less as the quality of professional inks improves. They happen most often in people that have sensitive skin, or when the artist packs an unnecessary amount of ink into the skin. This can also cause scarring.
There is no such thing as a "certified" professional tattoo artist. They must be licensed in most states, and in some be able to prove to health officials they have been through sterilization and communicable disease courses. No where do you have to prove that you are competent to actually do a tattoo. That is bull*hit made up by unethical and incompetent tattoo "artists" to lure people in. The only way to be sure that someone is competent is to see a tattoo on someone's skin and find out where they got it. Visit that place and ask questions regarding sterilization procedures and disposable needles. If you do not see the artist pull the needles and tubes out of sterile packages in front of you, RUN. If you see bars with needles still attatched sitting usually in the sharps container, and they say they break them off at the end of the day, RUN. They are probably lying and are reusing the needles. Make sure that the person who's work you saw that brought you there is the one you talk to about doing your tattoo. Just because someone works in the same shop does not always mean that they are as good, though a good shop will have standards, and artists of similar talent. Ask to see their photo album.
This is a business that you pretty much have to rely on how ethical your artist is, and unfortunately there are running about half and half ethical to unethical. If someone is evasive when you ask questions, DON'T TRUST THEM. If they are not evasive, just a little cranky, tattoo artists are just a cranky lot.
I don't mean to sound like I know everything, but this is a business I know a lot about. I owned a small shop years ago, and I have worked in tattoo shops as a piercer, floor person, receptionist, janitor, manager, and have spent 21 years with a highly ethical and talented tattoo artist.