Microbiology and Immunology On-Line Text Book - Sections on bacteriology, immunology, virology, parasitology, mycology. Links to other web resources - http://pathmicro.med.sc.edu/book/welcome.htm
Medical Microbiology by Samuel Baron, MD - An abstract of a book offers information about the traits of microbes and how they interact with humans. - http://gsbs.utmb.edu/microbook/