Best kind of mattress protector?

I've just moved into a new flat and the mattress is quite old and sketchy. What is the best kind of mattress protector to get that will protect ME from the bacteria in the old mattress (rather than the other way around!)? Most of the products I see are a mixture of cotton and polyester, but this doesn't seem like it will work effectively for what I'm looking for. Maybe vinyl? And does anyone know of where I can find the protectors that zip and cover the entire mattress, rather than the popular fitted style (oh, and at a reasonable price... I am a student!). Thanks for all the help, it's much appreciated!!

Here's a link for a vinyl cover that zips http://imperialtextile.com/campstuff/bed/Mattress/Bed-Mat.php I don't know anything about the company or the quality of the product but the person who stated that it would be hot & sweaty was right - if you could encase the mattress with the vinyl cover but then put a mattress topper on it to make it more comfortable & less "crunchy" from the plastic (another link - I know that you can find them for less - try Walmart, overstock.com, amazon.com - but this will give you an idea: http://www.rockymountainmattress.com/energia5lbtopper-p-54.html ).

If the mattress topper is too expensive, get a few cheap cotton/ploy Mattress Covers to put on top of the encasement so you'll be more comfortable when you sleep.

PurFlo SleepSystem Cot Mattress - Kiddicare

Vinyl Mattress Protector 6 Gauge - Zippered (9' depth) Twin Size $8.69 Our hospital grade vinyl mattress covers (Zippered Style) are odorless, waterproof, durable, and virtually noiseless! Zippered covers protect from spills, stains, bed bugs, dust mites, bedwetting and allergies. Retard mildew, mold and fungi with Ultra-Fresh - an anti-bacterial treatment. Easily wipe clean with a damp cloth. Our 6 guage extra thick covers provide the highest quality in durable, com...

Vinyl Mattress Protector 6 Gauge - Zippered (9 depth) Queen Size $12.99 Our hospital grade vinyl mattress covers (Zippered Style) are odorless, waterproof, durable, and virtually noiseless! Zippered covers protect from spills, stains, bedwetting and allergies. Retard mildew, mold and fungi with Ultra-Fresh - an anti-bacterial treatment. Easily wipe clean with a damp cloth. Our 6 guage extra thick covers provide the highest quality in durable, comfortable protection. 6...

Premium Waterproof Mattress Cover - King Size $46.95 Don't let an accident spoil a good mattress or pillow. Our Premium mattress, duvet and pillow covers provide a healthy sleep environment for your entire family. A 100% natural cotton surface provides comfort and absorbancy, while the waterproof layer repels body fluids and moisture. The waterproof, yet breathable fabric allows vapors to easily breathe through the protector. The protector protects ...