Your question is a little vague, so I’m going to answer multiple questions, some of which may not apply to you but hopefully you’ll find the answer your looking for (as will anyone else reading it.) If you are disease-free, meaning you’ve been tested and found free of all sexually transmitted diseases (STDs), most importantly HIV-AIDS, then you can’t infect your girlfriend via anal sex. On the other hand, because the anus is not made to have sex, very often little tears occur when having anal sex, and so if you have any STDS the odds of infecting her are higher than when having vaginal sex. Of course if you have an STD, you could also infect her during vaginal sex. Condom use can help prevent the spread of disease, in both vaginal and anal sex, but condoms can break or fall off, and some STDs (such as herpes) can be transmitted even if you are wearing a condom.

If you are disease-free and engage in anal sex and then put your penis inside your girlfriend’s vagina without either cleaning it first or putting on a new condom, then you would be introducing into her vagina germs from her anus which don’t belong there and are likely to cause an infection. And if there is any fluid that leaks out of her anus, including your ejaculate, you should both be careful that it doesn’t run into her vagina. In other words, take every precaution to make sure none of the germs in her anus get into her vagina. Now you might be wondering about your penis, if you weren’t wearing a condom. You too could pick up an infection, but a vagina provides a better environment for these germs to grow and multiply than does a penis, which is why even more precautions have to be taken with regards to the female than the male.