More likely he was already infected, and was using this "freak out" as a pretense to destroy the base's ability to warn everyone else, and simultaneouly make the others sequester him (as opposed to sequestering himself, which would arouse suspicion). It's shown later in the film that he had already been tunneling under his shed for some time, and was stealing parts to make himself a small aircraft for his escape. Very, very nefarious. The subtle plot layering and slow drip of information to the audience is one of the reasons this film maintains such a strong following to this day, as it rewards multiple viewings with more insight and opportunities for speculation.

Possibly...however, his actions present a much darker, chilling view if we consider that he may have already been infected at this point. If so, then destroying the other survivors' ability to contact and warn the outside world, setting himself up to be isolated by the group (so he can operate without observation) and also providing himself with more metal scrap and wiring to build his little escape ship...suddenly good ol' Dr. Blair doesn't seem very self-sacrificing.

"It's the right thing to do!" 😂