According to a study by The Barts and The London NHS Trust, up to a third of the weight of your pillow could be made up of bugs, dead skin, dust mites and their faeces.
And the condition of pillows in hospitals is even worse as they are the ideal breeding grounds for undesirables ranging from the superbugs MRSA and C.diff to flu, chicken pox and even leprosy, the scientists said.
But the problem of bugs in pillows is not confined to hospitals, said study author Dr Arthur Tucker, a clinical scientist at St Barts and the London Hospitals.
While some of the bugs will only be found lurking in hospitals or in tropical climes, others will be making themselves at home in the comfort of your bed, he said.
"People put a clean pillow case on and it looks and smells nice and fresh but you are wrapping up something really nasty underneath," he was quoted as saying by the Daily Mail.
Bacteria feast on these and multiply on and inside the pillows -- some of which will never be washed, he added.
The warnings came after the researchers studied the "health" of hundreds of pillows used by patients in hospitals run by Barts and the London NHS Trust.
They found high levels of "living" contamination on the outside of the pillows. In some cases, rips and tears meant that the germs had found their way into the filling.
Some pillows were contaminated with the E.coli stomach bug. Others contained germs that can cause respiratory and urinary tract infections.
Many hospital-issue pillows had more than one million Staphylococcus hominus per millilitre -- the bug can cause severe infections in people with weakened immune systems.
Describing the level as a "bio-hazard", Dr Tucker said "the presence of these bugs means that they can and will be passed to patients".
The researchers compared the bugs growing on and in standard hospital pillows with those found in anti-bacterial versions.
They found SleepAngel pillows, made by the Irish firm Gabriel Scientific, were less likely to be contaminated on the surface than the standard NHS ones -- and none tested positive for germs inside.
Duncan Bain, technical director of Gabriel Scientific, said: "If you had to come up with a medium to cultivate bacteria, besides a Petri dish with agar [a gelatinous food], a pillow is pretty much as good as you can get.
"It is a wet sponge that absorbs bodily fluids of various kinds providing nutrients. It is kept at the ideal temperature by the warm body lying on top."
Meanwhile, leading bacteriologist Professor Hugh Pennington urged people not to worry about their pillows, as they will simply contain the bugs they have already.
"There is plenty of opportunity to spread bugs partner to partner without pillows," he said.
