Monkeys that have recovered from psittacosis pneumonia have an increased resistance to infection with the virus and possess neutralizing antibodies in their sera.
Large amounts of active psittacosis virus can be introduced intravenously and intramuscularly into monkeys without the production of a serious infection such as pneumonia. Relatively small amounts of virus introduced intratracheally into monkeys usually lead to psittacosis pneumonia.
Monkeys vaccinated intramuscularly with unattenuated psittacosis virus have an increased resistance to the active agent and possess neutralizing antibodies in their sera.
The intramuscular introduction of active psittacosis virus in moderate amounts into human beings is relatively harmless, and repeated inoculations lead to the appearance of neutralizing antibodies in the sera of vaccinated individuals.