House dust mites, known as Acarids are less than 0.3 millimeters long (making them invisible to the naked eye) are similar to spiders. They feed on a diet of flakes which are shed from human skin and from microscopic fungus or mould which commonly grows on mattresses.
Bedding makes an ideal environment for these creatures to live as there is an abundance of shed skin. This coupled with a temperature of 20-30 degrees C and the relative humidity created by body sweat results in an ideal environment for them to breed in abundance. So much so that it is estimated that one gram of dust taken from a mattress could contain anywhere from 2,000-15,000 Acarids.