Billy stepped on a beehive and got stung all over; now when he hears buzzing sounds, his blood pressure increases and he sweats.

Phobia.

Explanation:

Phobia may be defined as a type of anxiety disorder that are marked by the excessive and persistent fear of the situation or any object. This phobia might result in the sudden fear.

Melissophobia si known as the phobia of the bees. The individual develop a persistent fever on seeing the bees. As billy was once stunged by the hive. She has develop the extensive fear against the bees.

Thus, the answer is phobia.