People tend to confuse the terms mattress cover and mattress pad.  A mattress cover is part of the mattress.  It encases the latex layers. 

A mattress pad is an accessory that provides protection to the mattress.  It is about ¼” thick and goes over the mattress like a fitted sheet.  It is in your best interest to put a mattress pad directly over the mattress to protect it from stains and spills from various sources.