Contagious picks up right where Infected left off, featuring most of the same characters and dealing with much the same basic issue: an alien species is attempting to create a portal from their world to ours, that they might invade. To do so, they need workers, and these workers grow from human bodies (turning the host paranoid and violent before finally killing them entirely during the 'birthing' process). The new twist introduced in the second book is that certain humans who've been infected with a new strain do not grow workers, but instead simply fall under the control of the entity that's been generating all of this chaos ... or, more accurately, first fall under its control and then under the control of a single human host who's grown too powerful for that entity to dominate.
These new infected can spread the infection, either through a kiss (their tongues are the vector), or, as we later find out, through other means entirely. This makes things much more difficult for our protagonists, as it means the infection can travel far and fast, and to unexpected people, who are almost immediately overwhelmed by it.
Contagious is, like Infected, a book filled with visceral violence and fast action. It improves on Infected in a number of ways, however, including improved characterization and dispensing with a certain idiomatic phrase that the first book drove into the ground. It also contains less vivid descriptions of bodily mutilation, which made it feel less like Night Shift-era Stephen King to me. Instead, Sigler ups the actual science to even greater levels, which I liked a lot.
Overall, Contagious feels to me like a writer who's growing more comfortable in his own style. The ultra-short chapters and casual tone help make it a fast read (it took me a while to get through it only because I accidentally left it at home during a vacation), and I liked that none of the characters come through this one sparkling clean; everyone makes tough decisions and gets some dirt under their fingernails.
Sigler is a little too into "all guys be like this" for me, at times, but I didn't find that it drew me away from the story much. I did have a hard time getting into things at first, as there weren't any characters I particularly liked, but what Sigler does with both Dew Phillips and Perry Dawsey worked for me, and in particular I found the ending satisfying and at least mildly touching. An unexpected but oddly pleasant (particularly given what happens in the book's final ten pages) way to wrap things up.
I started out thinking this was a three-star book, then it grew on me, and the ending chapters really helped take it up to four.