In the film 28 weeks later, how does the father get infected??!

In the film 28 weeks later, how does the father get infected??

my sister and i are having a debate... she says that the father gets infected by the mother biting him, thought i say that it is impossible since she is only a carrier. i say that it is through their kiss that the father gets infected. can you guys help us out?

3 weeks ago
to add on my sister says that while they kissed the mother bit him

While exchanging a kiss, Don becomes Infected. No bite.