A sudden fever in a 2-year-old baby with no symptoms is commonly caused by a reflexive emergency response of the body after spending too much time in a high heat or relatively cold environment. Be careful not to leave your baby in an environment that is too cold or too hot. In addition, it is also seen in the baby’s body with the invasion of pathogens, common pathogens are viruses, bacteria, mycoplasma, chlamydia, rickettsia and even fungi. The pathogens invade the baby’s body and cause an inflammatory response that leads to fever. Blood tests can be performed to find possible foci of infection, identify the cause, and treat the symptoms.