In their epidemiological studies on the epidemic of Japanese B encephalitis that broke out in Okayama Prefecture for past 10 years (1950-1959), the authors obtained the following results.
1) Every year's morbidity rate in Okayama Prefecture was higher than that in all Japan, and above all it was most prevalent in 1950 and 1956.
2) As for the district of prevalence, patients came out most numerously at the west-south part of the prefecture surrounding Kurasiki City and least at the west-north part of the prefecture.
3) As for the season of prevalence, 24th of August was a peak of the prevalence and there is about one week's lag between both peaks in southern and northern district, showing what we call “Tendency towards north”.
4) As for the sex, male morbidity rate was higher than female one, and on the contrary female lethality incidence rate was higher than male one.
5) As for the age, children below 10 ages and old agers above 60 are apt to suffer from the disease and the child patients in the southern district are far more than those in the northern district.
6) The ratio of the morbidity rate of the persons who were vaccinated to that of the untreated persons is about 1/2.4-1/4.4 every year, indicating the remarkable effect of the vaccination.
7) 10 Japanese B encephalitis strain viruses (Okayama52A-59J) were isolated from the brains of the encephalitis patients for past 10 years and these strains have been identified serologically with Japanese B encephalitis Nakayama strain.

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Through the collecting investigation of mosquitoss by light trap in Okayama prefecture in 1960, usually observed phenomenon that the incidence of Japanese B encephalitis follows the maximum peak of Culex tritaeniorhynchus about 2 weeks or 1 month later was not clearly observed.
A suggestion that the outbreak of Japanese B encephalitis begins south and spreads to north was not observed.
A prodigiously great number of C. tritaeniorhynchus were caught on each collection at some restricted places at Kurashiki City, where they might hibernate in Winter.

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