Salicylic acid (SA) is absolutely required for establishment of acquired resistance in non‐infected tissues following localized challenge of other leaves with a necrotizing pathogen. Although not directly responsive to SA, or induced systemically following pathogen challenge, the expression of defence gene promoter fusions AoPR1—GUS and PAL‐3—GUS after wounding or pathogen challenge could be enhanced by pre‐treating tobacco plants hydroponically with SA, a phenomenon designated ‘potentiation’. Potentiation of AoPR1—GUS wound‐responsiveness was also demonstrated locally, but not systemically, in tobacco tissue exhibiting acquired resistance following infection with either viral or bacterial pathogens. Potentiation of wound‐responsive expression by prior wounding could not be demonstrated. In contrast, potentiation of pathogen‐responsive AoPR1—GUS expression was exhibited both locally and systemically in non‐infected tissue. The spatial and temporal exhibition of defence gene potentiation correlated directly with the acquisition of resistance in non‐infected tissue. Pathogen‐responsive potentiation was obtained at about 10‐fold lower levels of salicylic acid than wounding‐responsive potentiation in AoPR1—GUS tobacco plants prefed with salicylate. These results may explain the failure to observe systemic potentiation of the wound‐responsive defence gene expression. The data suggest a dual role for SA in terms of gene induction in acquired immunity: a direct one by induction of genes such as pathogenesis‐related proteins, and an indirect one by potentiation of expression of other local defence genes (such as PAL and AoPR1) which do not respond directly to SA but become induced on pathogen attack or wounding.

Recurrences of typhoid fever are uncommon, and the recent literature is disappointingly uninformative about them. Despite his vast experience of the disease, Bretonneau (1829) reports that he had never seen a recurrence. Eichhorst (1888) states that of 666 consecutive cases of typhoid fever in Zurich between 1884 and 1886 twenty-eight gave a history of a previous attack; he observes that the severity of the second attack was independent of that of its predecessor, and that the mortality rates of first and second attacks were about equal; the average interval between attacks was about 10 years, and in only one patient was it less than a year. Curschmann (1902) states that the incidence of second attacks in 1888 patients in the Hamburg epidemic of 1887 was 24 %, and emphasizes that clinical records of the first attacks were often available for verification. He quotes other series with 18-2-2 % of second attacks. His general observations agree with those of Eichhorst. Sawyer (1914) reports that of 93 victims of an explosive outbreak oftyphoid fever in the United States 14 patients had experienced a previous attack. Gay (1918) reviews the literature and quotes numerous estimates of the frequency of recurrences, almost all based on patients' past histories.In general, these estimates vary from 1 to 4 %, but are as high as 15 % when the character of the outbreak suggests a large infecting dose. He states that second attacks are usually less severe than first attacks, and that recurrences usually occur in epidemics. Many of the estimates quoted by Gay are open to serious question as they were made at a time when the diagnosis of typhoid fever could not have been confirmed bacteriologically or serologically, and typhoid must often have been confused with paratyphoid and even with other continued fevers.Numerous authors (Dowling, 1948;Kolmer & Tuft, 1941;Rolleston & Ronaldson, 1940;Stuart & Pullen, 1950;Top, 1947;Zinsser & Bayne-Jones, 1939) agree generally with Gay and give no new figures. The general impression conveyed by the literature is that an attack of typhoid fever confers a considerable degree of immunity but that recurrences occasionally take place.Repeated exposure to infection by Salmonella typhi cannot be common in modern civilized communities, and this must greatly restrict the incidence of second attacks irrespective of the degree of immunity conferred by one attack. Two outbreaks of typhoid fever have recently occurred in the same community within a period of .5 months thus exposing a large number of individuals to the risk of infection on two occasions; several contracted typhoid fever twice. This unusual event forms the basis of the present report.

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SUMMARYIn the Cambridge area, 143 patients infected with Corynebacterium haemolyticum were found during the period May 1967 to December 1974. The pharynx was the commonest site of infection and 100 of the 137 pharyngeal infections were in patients aged between 15 and 25 years. Such patients usually had a sore throat; about half of them also had a maculopapular rash. The organism and the clinical features which we have come to regard as typical of this infection are briefly described.

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