Toxoplasma gondii and cytomegalovirus: mixed infection by a parasite and a virus.

Human fibroblasts infected in vitro with cytomegalovirus are relatively resistant to infection by Toxoplasma gondii during the first 4 days of virus infection. After 5 days, however, the cytomegalovirus-infected cells become susceptible to the parasites. The toxoplasmas replicate in paracentral rosettes surrounded by host cell mitochondria. This growth configuration differs from that seen in human fibroblasts infected in vitro with toxoplasma only but resembles the pattern seen in doubly infected cells found in human necropsy tissue.