In 2011, after the end of the 2009 new influenza epidemic, four diseases, Kawasaki disease (KD), hand-foot-and-mouth disease, erythema infectiosum, and aseptic meningitis, occurred simultaneously in Tokyo, but in 2024, after the COVID-19 pandemic end

Following the largest number of seasonal flu cases ever occurred in Tokyo, the new influenza AH1pdm09 pandemic that broke out in Mexico also happened in the summer and fall of 2009 and ended without seasonal flu outbreak in the spring of 2010. The year 2011, after the end of the 2009 new influenza pandemic, was the second highest pollen dispersal year in the history of Tokyo, when the number of cases of Kawasaki disease (KD), hand-foot-and-mouth disease (HFMD), erythema infectiosum (EI), and aseptic meningitis (AM) per sentinel increased simultaneously. The year 2013 was the fourth highest pollen dispersal year in history, followed by an increase in KD, HFMD, and AM, but a decrease in EI. However, in the first half of 2015, EI was very prevalent. In 2018, when the third highest pollen dispersal in history occurred, the number of patients affected by KD was the largest ever and the prevalence of EI was significant, while the extent of HFMD and AM cases was moderate. The COVID-19 pandemic began in early 2020 and the epidemic lasted for several years, with a low point in the 40 weeks of 2023. Ninety percent of the pollen from 2023, the fifth highest pollen dispersal year in history, also flew in 2024, making 2023-2024 the two consecutive year of high pollen counts, when the number of KD patients in week 22, 2024 exceeded the number of patients in 2018. The increase in the number of KD patients in the phase of the annual fall/winter period, which began around October, has been suppressed by the impact of the sudden increase in the number of patients with Mycoplasma pneumonia, which has been sustained and increased. A sudden increase in the number of AM patients occurred in a few weeks during this period. HFMD was also marked by an increase in the number of affected patients along with EI, and unusually began to increase again at week 34 and began to decrease at week 42. EI still continues to increase at week 48. In 2024, after the COVID-19 pandemic ended, the incidence of KD was suppressed by the Mycoplasma epidemic, and three diseases other than KD became prevalent simultaneously, and the simultaneous outbreaks of the four diseases observed in 2011 after the end of the H1N1 influenza did not recur.