The Judas Strain by James Rollins
This takes over the #1 slot with me among Rollins novels: it improves on City of Bones with tighter, better-focused action and, I think, more sound science.
What appears to be a natural phenomenon at Christmas Island in the Pacific - a kind of algal bloom that's more deadly than any seen before - turns out to involve the nefarious Guild, a (real) historical mystery going back to Marco Polo, and the quest for the ultimate biological weapon. Some of the "wisdom of the ancients" stuff is still farfetched (they knew about DNA how?) but this novel only relies on it enough to make the story work. The biological science is scary, very scary, and plausible. Rollins works in known parasites like the liver fluke that go through multiple host species and posits a virus with a similar lifestyle that, like some parasites living on insects, rewires the host (human) body to serve its needs. (One such ant parasite, a fungus, was the genesis for the X-Files episode "Firewalker.") I'm hardly an expert on virology or infectious disease, but the whole thing struck me as involving reasonable yet terrifying extrapolations.
The technology throughout this novel works fine: a last-chapter connection between continents on a low-power device initially struck me as absurd, but then I started working it out, and realized you could do it, and do it clandestinely, by bypassing the usual military/intel satellites and hiding it in a civilian system like ARGOS. That was reassuring, as I sometimes feel Rollins treats global communications almost as magic. The details of architecture and history are everywhere convincing, and the heroes have to rely heavily on their brains as well as their guns and gadgets to solve the mystery. Also, one of the early chapters wins some kind of ingenuity award for the cleverest use of a natural "weapon" to take out bad guys. Rollins usually works in some cryptozoology, which in this novel appears in the form of a new but not unlikely species of killer death squid.
Overall, this is the best-written of the Sigma series, one I had an easy time following but a hard time putting down.