Semen isn't infectious, period, because semen isn't a parasite or virus. Rather, it's a fluid that can -- when viruses or parasites are present -- be a way for viruses or infections to be transmitted.
Most STIs can't be spread passively, like by fluids being left on floors or towels. Otherwise, what you're asking really depends on a) if the semen in question is someone's who does have an active infection of some kind, and if so, then b) what kind of infection it is. It sounds like you could use some basic information about STIs in general, so you can see how some can be transmitted.
I'd also suggest looking at the general risk assessments for sexual activities here:
Can I Get Pregnant, or Get or Pass On an STI From That?
For a briefer reference, you can take a look at this:
STI Risk Assessment: The Cliff's Notes
You can find and read short summaries of info about all current STIs if you use this tag:
http://www.scarleteen.com/tags/sti_files
On the whole, as the receptive partner for oral sex, the STIs you're typically at risk of will be Herpes, Chlamydia, Gonorrhea, Hepatitis, Syphilis and HPV. HIV is a technical risk, but not a likely one for a receptive partner.