Well, it depends on what you mean by "infection".
One thing for sure is that it is the BEST way to get infected with the deadliest STDs, namely HIV. Of course many other STDs can be transmitted through this practice: hepatitis, herpes, gonorhea, clamydia, etc. The list is long.
Such a practice should only be practice in a monogamous relationship where both partners have been well tested. Even though, it can still be risky if one of the partners stray...
If you were asking about other types of infections unrelated to STDs, anal sex can cause these too. Particularly rough anal sex can lead to some nasty problems in the anus/rectal area. From my understanding, it is not caused by semen, if it was women would get vaginal infections when men ejaculated in their vaginas.
Infections in the anal/rectal area are caused by small fissures or tears in the skin (usually internally) which get infected. These often lead to abcesses and other quite severe complication. Considering which area this fissure or tear occurs it's no surprise that an infection can be the result.
I have been lucky never to have experienced such an infection myself as I have partaken in anal activities. I have known two acquaintances that have experienced this type of infection and related their experiences with me. Both were escorts, one male (gay) and the other female.
It's not that common but it isn't a rare occurance either.
Is there a doc in the house to address this issue?
My 2 cents,
Jackie