nvasive bacterial pathogens, the Chlamydiae know us very, very well. The Chlamydiae learned to parasitize eukaryotic cells half a billion years ago by reprogramming cellular functions from within. In humans today, chlamydial infections are responsible for a range of ailments from sexually transmitted infections to atypical pneumonias to chronic severe disorders such as pelvic inflammatory disease and atherosclerosis. The Centers for Disease Control says that Chlamydia trachomatis is the most common sexually-transmitted infection in the US, with three million new cases a year.

Chlamydia gets around because it knows its hosts so well. It's an "obligate intracellular parasite" which means that it relies on its eukaryotic host for everything from reproduction to synthesizing ATP, all while living inside a membrane-bounded vacuole that provides a protected, fertile environment for the bacteria to grow and multiply. Because lipid acquisition from the host is necessary for chlamydial replication, these pathogens are essentially lipid parasites. So, to add insult to injury, Chlamydia apparently lives on our fat.