The summary Japan has reported a record number of cases of streptococcal toxic shock syndrome, a rare and dangerous bacterial infection caused by Group A strep bacteria. The U.S. similarly saw serious Group A strep infections — including STSS — reach a 20-year high in 2023. Experts think a post-pandemic rebound of bacterial and viral infections could partially explain the trend, but many unanswered questions persist. A record-breaking rise in potentially fatal infections in Japan is bringing attention to persistent, unanswered questions about the group of bacteria behind the illnesses. The rare infection, streptococcal toxic shock syndrome (STSS), is caused by Group A strep bacteria, the same type that causes strep throat and scarlet fever. In rare cases, Group A strep can enter deep tissue or the bloodstream, as is the case with STSS. Up to 30% of STSS infections are fatal: The condition usually starts with a fever, chills, muscle aches, nausea or vomiting but can become li