There's no need to be "MEEC" with your differential diagnosis for aseptic meningitis! Here's a mnemonic to help you out.
Enterovirus (like polio)
If an infant comes in with a stiff neck, altered mental sensorium, irritability and high fever, get some CSF. If it's clear and fluid analysis shows elevated protein, but no decrease in glucose and high lymphs... think aseptic meningitis. Bacteria would make it cloudy and eat up the glucose.
Meningitis is one of those critical ER diagnoses. Don't hesitate or the baby could die! Unless its viral, then there's not much you can do but provide supportive care until it resolves. :-\ Herpesviruses and varicella can be treated with antivirals.