"Peeps" is a nickname given to people who are "Parasite Positive" in this young adult novel by Scott Westerfeld. This particular parasite makes people cannibalistic and also to repel the things they loved before they lost their minds. Cal Thompson is living in New York City, infected by the parasite but - luckily - immune to its effects, like a carrier. He is recruited by a government organization that is trying to contain the disease, and learns way more than he ever wanted to know.
One unique feature of this book is the interspersed non-fiction chapters explaining the mechanisms by which various real-life parasites infect their hosts, like trematodes and Toxoplasmosis gondii. It is... not recommended for hypochondriacs.
There is discussion of sexual activity, because Cal does contract the parasite sexually, but it is not explicit. Perhaps the material is better suited to older teens.