Ok, so here is my first summary. If you can think of anything to add let me know!
This book begins with our main character, Kaelyn, writing a letter to her bff Leo, who she got in a fight with right before she moved to the mainland from her island town and hasn't spoken to since (although she misses him terribly). She has since moved back to the island, but Leo is on the mainland at his school so she still can't talk to him. Kaelyn goes to one of her friend's house one day to work on a school project and ends up leaving because her friend's dad is saying things that are inappropriate and weird. Kaelyn's dad is a doctor at the local hospital on the island and after a few weeks discovers that this strange behaviour is a new virus they have dubbed "the friendly virus" because it makes ppl lose their inhibitions and want to be around other ppl, therefore spreading the contagion which has symptoms that go from sneezing/itching, to the inebriated state, and then finally death (pretty smart virus huh?)
Later, Kaelyn goes to Leo's gf house and they start scavenging for medical supplies and food in the currently uninhabited summer homes but Kaelyn sees a dead body in a bed and refuses to go back, although her and Tessa remain friends from then on. Things worsen as more people die, and the government quarantines the island. This is the beginning of the end. Groups rise up who begin to loot and destroy buildings, but Kaelyn joins a group with a leader named Gav who is kind hearted and helps distribute food to the folks who are still alive and uninfected but afraid to come out of their homes.
When things really get bad, the government deserts the island and doesn't even do food drops for the people anymore, so ppl are going hungry and getting more vicious, even as more ppl die. Kaelyn contracts the virus, but ends up surviving, which turns out to be because she was exposed to a less mutated form when she was sick a year earlier and has developed a type of immunity. Her mother becomes sick and dies, along with her uncle (although he wasn't sick: when the gov had the island under quarantine and the riots started they shot and killed him), and Kaelyn takes in her niece Meredith (who I think is 10 or 11, I don't really remember).
Near the end of the book, Kaelyn's dad is killed in a riot by a group who are going to burn down the hospital to rid the island of the infected, and Meredith gets the virus. Kaelyn goes to the hospital to speak with the only doctor left, her fathers friend Nell, and Nell takes some of Kaelyn's blood and gives to Meredith to try to cure her.
At the end of the book, we see that Meredith is starting to get better, and Kaelyn has begun to fall for Gav. Tessa is now staying with Kaelyn at her house since looters destroyed her home and greenhouse. We go out with seeing a ferry coming across the harbor towards the island with people aboard (including Leo), so it leads you to believe the mainlanders are coming to rescue those still left on the island.