In the US, most deaths are attributable to chronic afflictions, such as heart disease and cancer. Typically the medical community has attributed these diseases to accumulated damage, such as plaque formation in arteries or mutations in genes controlling cellular replication. This view is changing. Scientists are now beginning to recognize that many of these chronic illnesses are due to microbial infections. A recent report in the American Journal of Psychiatry suggests that schizophrenia, a mental illness leading to errors in perception, is associated with the pathogen, Toxoplasma gondii. "Our findings reveal the strongest association we've seen yet between infection with this very common parasite and the subsequent development of schizophrenia," study investigator Dr. Robert Yolken of John Hopkins Children's Center in Baltimore said in a statement.  Toxoplasma gondii, a parasitic protozoa usually carried by cats, is an interesting microbe, having been associated with behavioral changes in humans and rats. Other diseases are also showing microbial connections. The most famous example is Helicobacter pylori, the unequivocal cause of peptic ulcers and suspected agent of gastric cancer. Human papillomavirus is associated with 90% of cervical cancers. Hepatitis B is linked with 60% of liver cancers. Some diseases look suspiciously like infections, such as Hodgkin's disease, multiple sclerosis, juvenile onset diabetes and Crohn's disease. In evolutionary terms, these speculations make sense. Microbes exist to pass on their genes, and they may have evolved ways to "cryptic" rather than "conspicuous". Paul Ewald divides diseases into three categories: environmental, genetic and infectious. Environmental diseases are acquired from toxins such as those in cigarettes and pollution. Genetic diseases are those caused by errors in replication and development. Ewald reasons that, if diseases are too common to have arisen by random mutation and too ruinous to have survived natural selection, and if it is not environmental, then it must be infectious. By being cryptic, these infectious diseases enhance their spread by increasing the odds of transmission. Toxoplasma is one of Carl Zimmer's favorite parasites. Photo: Toxoplasma gondii from Wikicommons.