The difficulties attending the diagnosis of mild types of typhoid fever have made the study of the duration of immunity conferred by an attack a perplexing one. The differentiation of typhoid-like, paratyphoid, colon and paracolon infections is by no means easy, even for those so fortunate as to have at their disposal ample facilities for conducting blood culture and agglutination tests against strains of all the bacteria mentioned.
Typhoid simulating mild types of meningitis, early tuberculosis and many pneumonias with delayed resolution, likewise malarial fever (so-called) is frequently overlooked. In our hospital wards one rarely hears, in taking the history of a typhoid case, reference to an earlier attack. Most writers agree that, as in many other acute systemic infections, one attack is sufficient to produce a probable, although not absolute, immunity against later attack. Dreschfeld's figures, based on 2,000 cases in the Hamburg General Hospital, show that only