Here are some statistics about allergies and dust mites

The World Health Organization ranks allergies fourth on a list of the world's most common chronic conditions—they ranked sixth 20 years ago. They could rise to third place over the next decade, because allergies are increasing steadily worldwide.
 
Ten years ago, WHO predicted that one in two people would suffer from allergies by 2010. The estimate was only two years off—current projections say we could reach that number by 2012.
 
About half of allergy sufferers are sensitive to dust mite allergens. (Clin Exp Allergy, 2004; 34:597)
 
A female dust mite can lay up to 300 eggs during its short 80-day lifespan and excrete up to 200 times its body weight in fecal matter. (Compétences Médicales Supplément  No. 4, 1996)
 
Dust mites can trigger asthma-related symptoms and cause asthma in children who have never previously displayed asthma symptoms (United States Environmental Protection Agency, 2008).
 
According to WHO, respiratory allergies are among the most prevalent chronic diseases in children.
 
Dust mites account for 70% of all respiratory allergies.
 
Food allergies are increasingly common but are not as widespread as dust mite allergies, which have become a full-blown public health problem. From 60% to 80% of asthmatic children are sensitive to household dust mites (Dr. Étienne Bidat and Christelle Loigerot.  2003. Les allergies de l'enfant, Ed. Milan, Toulouse).
 
Most (80%) allergic asthmatics also suffer from allergic rhinitis and 40% of untreated rhinitis cases end up developing asthma.
 
Allergic rhinitis affects 10% to 25% of the world's population and is on the upswing.
 
Patients suffering from allergic rhinitis are three times more likely to develop asthma.
 
Allergic rhinitis affects 400 million people worldwide, half of whom also suffer from asthma.
 
Doctors have diagnosed asthma in more than 2.3 million Canadians, i.e., 8.4% of adults (Statistics Canada, 2009) and 13.4% of children (Garner, 2008).
 
Currently, dust mite allergies are the number one cause of bronchial asthma in the world.
 
In children who are sensitive to dust mites, exposure to just 10 micrograms of Der (dust mite allergen) can trigger a severe asthma attack. Effective elimination measures ensure that sensitized children suffer significantly fewer asthma attacks.
 
Allergic disease can be prevented by decreasing allergen density in the genetically predisposed (atopic) subject's environment. (Éditions Scientifiques et Médicales Elsevier SAS, 2001)
 
A German longitudinal MAS (multicenter allergy study) that began following a cohort of 7,609 children in 1990 found that sensitization to household allergens, which increased through age six, is more prevalent in northern regions. The study demonstrated a link between the presence of household allergens and the subsequent level of sensitivity—so early heavy exposure to household dust mites is a clear risk factor.
 
To get an idea of the impact of dust mites on the incidence of asthma in 0–18 year olds, keep in mind that 70% of asthmatics are sensitive to dust mites, as opposed to 20–25% for other common allergens such as pollen and animal dander. In other words, early application of dust mite elimination measures could reduce allergic asthma by at least 70% in genetically predisposed (atopic) children. (Compétences Médicales Supplément  No. 4, 1996)
 
Although it's impossible to entirely eliminate dust mites, simple measures can greatly reduce their numbers. The most effective way to inhibit their growth is to lower the relative humidity (Fernandez-Callas et al., 1995; Platts-Mills et al., 1989). They can't survive in relative humidity below 50%  (Fernandez-Caldes et al., 1995). In winter, when the outside humidity is low, better ventilation can help reduce indoor humidity due to human activities (Platts-Mills et al., 1992).