The third day. According to a query on the Youlai Doctor website, the third day of the severe stage of hand, foot and mouth disease in adults is the most dangerous. Patients experience symptoms such as lethargy, poor sleep quality, poor energy, decreased appetite, tremors in the limbs, and unsteady walking. Hand, foot and mouth disease, also known as "hand, foot and foot syndrome", is an acute febrile and rash infectious disease caused by enterovirus.