Platelets are a type of blood cell that stops bleeding. However, they are also involved in the pathologies of arterial thrombus formation. Although there are various causes of thrombocytopenia, inflammatory stress, such as viral or bacterial infection, rapidly decreases platelet production. Studies using mice have shown that inflammatory platelets are produced by a pathway different from normal under inflammatory stress. Furthermore, scientists have raised the possibility that a decline in platelet count and the production of inflammatory platelets may contribute to the sequelae of COVID-19 infection (long COVID).