Sources

pollens, which originate from plants
viruses, which are transmitted by people and animals
mold
bacteria, which are carried by people, animals, and soil and plant debris
household pets, which are sources of saliva and animal dander (skin flakes)
droppings and body parts from cockroaches, rodents and other pests or insects
viruses and bacteria
The protein in urine from rats and mice is a potent allergen. When it dries, it can become airborne.
Contaminated central air handling systems can become breeding grounds for mold, mildew and other sources of biological contaminants and can then distribute these contaminants through the home

Many of these biological contaminants are small enough to be inhaled.

Biological contaminants are, or are produced by, living things. Biological contaminants are often found in areas that provide food and moisture or water. For example:

damp or wet areas such as cooling coils, humidifiers, condensate pans or unvented bathrooms can be moldy
draperies, bedding, carpet and other areas where dust collects may accumulate biological contaminants