Quite technical. If you get put off in the early chapters skip through to chapter 6, the book is less technical and more interesting at this point.

“Viruses that succeed in transferring to, and surviving in, a new host generally undergo a period of rapid adaptation. During this time, their fitness improves, meaning that they hone their survival skills so that they can infect, evade host immunity, and spread between the new hosts more efficiently.”

This is a terrific book if you don't mind detailed science writing. I suspect it will come off as tedious if you don't already think that viruses and cell biology are fascinating.