You've asked a preTty good question.
If you really want to be extra careful, you can. But usually its 99.9% safe. Whatever gets expelled from you, should not be able to pass through to her.
But its also good and safe to take extra measures!
julie75 answered Friday August 19 2011, 9:07 pm: If she's not on birth control and you want to be extra careful, you can. But, if you trust the condom to work, it's usually pretty safe. [ julie75's advice column | Ask julie75 A Question ]
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.