Final answer:
A virus is NOT considered a common allergen. Mold, tree pollen, and insect stings are common allergens that can trigger allergic reactions in some individuals.
Step-by-step explanation:
The correct answer is a virus. Viruses are not considered common allergens as they do not trigger an allergic response. Allergens are substances that can cause an allergic reaction in some individuals. Common allergens include mold, tree pollen, and insect stings. These substances can provoke an immune response in sensitive individuals, leading to symptoms such as sneezing, itching, and watery eyes.
Learn more about Common allergens