Hi bidzer,
Mattress covers aren’t designed to protect against body oils, stains, and spills (with a few exceptions usually with vinyl children’s mattresses) so a protector is always a good idea for a mattress.
There are a few tradeoffs involved with a woven cotton, wool, and the thinner breathable/waterproof types being the most common. Each offer a different combination of breathability, water resistance, and effect on the mattress.
A couple of the more popular choices in thin breathable/waterproof protectors are in post #4 here and there are quite a few other options (with some different “features”) in post #15 here.
Phoenix