Pillows are one of those creature comforts we just don’t think about all that much. Growing up, we had old pillows in our linen closet for years and years. Whenever I would have a sleepover party, I chose the pillow with the mysterious brown stain in the middle since it was the thinnest and packed easily. I didn’t stop to think about where that brown stain had come from. But maybe I should have. Turns out, keeping old, worn-out pillows can be bad for your health.

Pillows can harbor thousands of dust mites (and their excrement), dead skin cells and bacteria. Think about it. You’re breathing, sneezing, drooling or sweating directly on it for six to eight hours a night. Indeed, a 2005 study from the University of Manchester in England found as many as 1 million spores of up to 16 different kinds of fungi in some used pillows. If you have allergies, dust mites or fungi living in your pillows may exacerbate your symptoms.

"We know that pillows are inhabited by the house dust mite which eats fungi, and one theory is that the fungi are in turn using the house dust mites' feces as a major source of nitrogen and nutrition (along with human skin scales). There could therefore be a 'miniature ecosystem' at work inside our pillows," according to study author and University of Manchester professor Ashley Woodcock. Plus, "we put 20 gallons of sweat into our beds each year," Woodcock told The Seattle Times.

Keeping pillows clean

Standard synthetic-fill pillows can be thrown in the washing machine two to four times a year. Some machines have a designated pillow cycle. If yours doesn’t, use the delicate cycle instead. If you have a down-fill pillow, some say you need to dry-clean, but housekeeping expert Martha Stewart recommends throwing them in the machine on a delicate cycle (but only a machine without an agitator in the middle, since that can damage the pillow). She also recommends the following pillow-washing tips:

Use mild liquid detergent rather than powder, which may leave a residue. Launder pillows in pairs to keep your machine balanced. Run them through the rinse cycle twice the second time without detergent, to ensure they're rinsed fully.

Then, run it through the dryer on low heat (or the air-dry cycle if your machine has one) a couple times to be sure it’s completely dry inside. Take it out and fluff it a couple times mid-cycle to ensure all the stuffing is drying evenly. If any moisture is left, it could harbor mold growth, which you definitely don’t want in the place you rest your head every night.

If you have a non-machine washable pillow or if your washing machine has an agitator in the middle that could damage your pillow, consider this method suggested on the "Today" show:

If you have a memory foam pillow, they cannot be thrown in the washing machine and require special washing care.

The recommendation for how often to wash your pillowcase depends on which website you visit or expert you ask. But generally, washing your pillowcase once a week is wise. If you’ve been sick, wash them every couple days instead.

Making pillows last longer

Pillow covers, which are zippered covers that go in between the pillow and the pillowcase, will help keep dust mites and other bacteria from getting into the pillow itself and will help extend your pillow’s life. The cover should be breathable and moisture-repellent, says Woodcock, a professor at the University of Manchester and author of the aforementioned study. Wash the pillow cover every time you wash your sheets.

If you don’t have a pillow cover, it’s a good idea to replace your pillows (if they are synthetic-fill) every couple years. You can do the fold test to see if it’s time to replace it — if you fold it in half, let go and it pops back into place, it still has some life left. If it just sits there and stays folded, better to toss it. If you’ve invested in a good down-fill pillow or a latex pillow (which is all the rage for allergy sufferers these days) and wash it often to keep it fresh, it can last as long as 10 years.