This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
Debate about this arose after Jeanie Boulet was revealed to
have been infected with the HIV virus via her husband Al, who was
promiscuous during their marriage. Jeanie and Peter had a brief
affair during the early second season.
In the early third season, Peter was tested, and his test
came back negative. There seems to be some isolated confusion about
this on Usenet because we the viewers didn`t actually hear anyone
tell Peter, out loud, of his HIV-negative status; all we see is
Peter`s reaction while listening to an unheard voice on the
telephone. Peter subsequently tells Jeanie that his tests came back
negative. Some people believed Peter`s reaction to the phone call to
be sufficiently ambiguous to cause speculation that his tests were in
fact positive, and that he lied to Jeanie, but in my opinion, there
is no issue; watching Peter`s expressions and behavior with any
amount of attention should confirm that he`s relieved, albeit a
little stressed, as a result of that phone call.