I've got red blisters in my throat, what could the cause be?

It's a virus: Red blisters or "vesicles" in the back of the throat are usually a virus. A throat infection is called a "pharyngitis." Although a herpes virus can cause viral pharyngitis,, so can a host of other viruses. The symptoms are usually self-limited, but should definitely be considered contagious during the time they are present. Strep infections often present with white discharge, not blisters.