Some of these articles may be heavy reading, but they are well researched studies.
All papers sited below determine the high prevalence of dust mites and their alllergens in bedding, household fabrics and soft toys. All papers suggest covering bedding in dust-proof material is an ideal way to confidently control dust mites and bed bugs.
UK Nation Health Service: Dust mites health effects and control in the home.
Synthetic or feather pillows? Feather pillows often have tighter weave cases. While less dust mites can penetrate them, allergens are still present. Therefore, cover all pillows.
Dust mites in soft toys: How to kill dust mites in soft toys. Freezing soft toys, hot tumble dry or wash soft toys in a pillow case with eucalyptus oil. All these methods reduce dust mite allergens.
Allergic reactions; asthma, allergic rhinitis and eczema.
European Study: Dust mites are one of the greatest causes of allergies around the home.
Eczema, rhinitis (hayfever) and asthma in children caused by dust mites.
(eBay link to bed covers)