Pollen is the most common cause of seasonal allergic rhinitis, sometimes known as "hay fever."
Mold spores or fragments may be inhaled through the nose and into the lungs, where they cause allergic reactions or asthma.
Household pets, like cats and dogs, are the most common sources of animal allergens.
It's not the dust mite itself that causes trouble for people, but its shed skin and fecal matter. These substances bring misery to millions of allergy sufferers.