People are usually worried just about contracting the "monster" the "kitty" ie HIV, but there are also a plethora of other STIs that can be contracted:

Herpes, Gonorrhea, HPV, Hepatitis B, Syphilis, Chlamydia, Chancroid, and though statistically low, the possibility of contracting HIV exists.


This from the Center For Disease Control And Prevention (CDC)

http://www.cdc.gov/hiv/resources/qa/qa19.htm

Can I get HIV from oral sex?

Yes, it is possible for either partner to become infected with HIV through performing or receiving oral sex. There have been a few cases of HIV transmission from performing oral sex on a person infected with HIV. While no one knows exactly what the degree of risk is, evidence suggests that the risk is less than that of unprotected anal or vaginal sex.

If the person performing oral sex has HIV, blood from their mouth may enter the body of the person receiving oral sex through

the lining of the urethra (the opening at the tip of the penis);
the lining of the vagina or cervix;
the lining of the anus; or
directly into the body through small cuts or open sores.

If the person receiving oral sex has HIV, their blood, semen (cum), pre-seminal fluid (pre-cum), or vaginal fluid may contain the virus. Cells lining the mouth of the person performing oral sex may allow HIV to enter their body.