Familial cases of psittacosis: possible person-to-person transmission.

A 29-year-old woman with primary myelofibrosis developed severe pneumonia, and 20 days later her 31-year-old sister also contracted pneumonia. The first patient had been in contact with parakeets but the second patient had not been in contact with any birds. Psittacosis was diagnosed in both cases by microplate immunofluorescence antibody technique. Person-to-person transmission between the sisters was suspected to have taken place.