Indeed they are, and there are a number of plausible alternatives as to why that may be. Early speculations centered on the changes in our living situations and our households. Indoor pets have certainly become more prominent, carpeting and less outdoor air entry might lead to increased levels of dust mites, and maybe mold… but the most intriguing consideration of all is referred to as the “hygiene hypothesis” which proposes that our immune systems develop along two major pathways in that they are largely complementary in that when one goes up, the other goes down. One directed more at infectious illnesses and the other at more of an allergic response.
Modern life and medicine have provided many improvements: better sanitation, effective antibiotics, immunization… all tending to reduce our infectious burden. But here’s the rub: allowing a generation of more allergic responses. Many pieces of epidemiologic evidence support this concept. So when mom told you to “go wash your hands” it reduced the bacteria but may have actually let to a greater likelihood of greater allergic disease.