Asthma
Pets: animals give off chemicals called proteins which are  allergens; people can be allergic to them. These allergens can act as irritants and make someone's asthma worse and trigger an asthma attack. The proteins are in the pet's dander which is the dead flakes of skin that animals (and people) shed. They are also in their urine, feces, saliva, and  sebum which is made by glands in the skin called sebaceous glands. Sebum is what makes hair and skin oily. When dander, urine, feces, saliva, and sebum dry out their proteins can become airborne and breathed in. Some of the types of pets people can be allergic to are, dogs, cats, gerbils, hamsters, guinea pigs and pet birds.