I think Bob got bit when he was pulled under the water. At least thats what they want us to think.
Especially when he asked Sasha for "one more kiss" then went outside andf well you know what happened next.
Interesting things to think about,
If Bob was bit, will ther guys who are eating his leg turn? After all they would be eating tainted meat.
Also, In the comics, this happens to Dale instead and he tells them that he was bit and that they are now eating contaminated meat.
CBF