Final answer:
St. Louis encephalitis (SLE) is the condition called when symptoms of encephalitis are slow to appear and less severe, often caused by arboviruses carried by mosquitoes with most cases being asymptomatic.
Step-by-step explanation:
When the symptoms of encephalitis are slow to appear and less severe, the condition is referred to as St. Louis encephalitis (SLE). This form of encephalitis is relatively rare, with symptoms manifesting in fewer than 1% of infected patients. Many encephalitis cases are caused by arboviruses, which are viruses transmitted by insects such as mosquitoes. SLE is endemic to certain regions, such as the central United States, and can range from mild flu-like symptoms to severe complications like convulsions, coma, and even death, though most patients do not show any symptoms. Treatment options vary depending on the causative virus, and in some cases, effective vaccines are available.