Cccidiosis in chickens and other birds is very common and can be very problematic in backyard flocks. It is caused by a protozoa, which multiplies very quickly and can debilitate a bird in a couple of days.
A vet may prescribe Intradine or baycox; although the latter is dearer.
This link provides good information on the subject.
http://www.dpi.qld.gov.au/cps/rde/xchg/ ... A_HTML.htm
The following link gives pictures of which protozoa affects which part of the hen.