According to experts, mattress encasements should be washed after every two months. However, if you've fallen ill, we'd advise that you strip off the bed, including the mattress cover, and wash them to remove any lurking viruses and bacteria.  Similarly, if you share your bed with a pet or if you sweat a lot, you would want to wash your mattress protector monthly to remove fur and sweat build-up.  Not to mention, if you've spilled something on the bed, you'd want to wash it immediately, so it doesn't stain.