Actually in the final episode, Sheila bites Joel, but later when they talked about this incident with Dr. Cora Wolf, and Dr.Cora asks her if she broke his skin, she says no.
We know that if Sheila bites someone, the person turns into a zombie, but We are never really explained who the virus fully works in the series.
Sheila does wonder if she could have gotten it from the clams that she ate at the restaurant Japopo's. Getting the virus from shellfish would be a twist on zombie folklore, but that reference could be more of a sex joke than a plot point.
According to this source:
In the season one finale, Dr. Wolf (Portia de Rossi) offers a few suggestions including a mutation of the rabies virus or a resurgence of an ancient plague, but she said speculation was pointless.
Eric suggests in the premiere that most new viruses come from animals such as monkeys or bats.
We just have to wait until season 2 comes out and explains a little bit more =)