One somewhat popular hypothesis that pops up every now and then is the "V5" virus. Not as popular, but still seen is the "K-17" virus. This is going to be a short article. In short, neither one exsists. Both have been latched onto by some real vampire sites and touted as the "real deal". But they not. V5 began to surface after the British TV miniseries "Ultraviolet" was aired in 1998. The "K-17 virus" is similar, but appeared after the movie "Reign in Darkness" in 2002. The two viral hypothesis only became "fact" after misguided people who could not discern reality vs. fiction.
This is not to say that the hypothesis of a viral cause is completely fictional, which is discussed in the (viral.html) Viral Hypothesis article. Mearly that these two particular viruses are fictional in relation to vampirism.