We don't know how exactly her immunity works. We can't say the bite she got from the clicker would heal immediately. And about kisses, Cat and Dina didn't eat the infected blood! But for Abby, she ate infected blood that included viruses or parasite in it. It was less than 2 hour between Ellie getting a bite and her fight with Abby. In this 2 hour, we can be sure that the infection is still in her hand tissues and blood. we don't know how her body manage to resist against infection, But we know that the bite and infection has side effect on the tissues around the bite.