A flea is a tiny insect; to flee is to run away. Remember that flee has two e's, like the two e's in escape, while flea has an a, like animal.
The core difference
Flea is a jumping insect, while flee is a verb meaning to run away from danger.
- flea — a tiny, wingless, jumping insect that feeds on blood: The dog kept scratching because of fleas.
- flee — to run away from danger: Thousands of residents fled the city before the storm hit.
How to tell them apart
Flea is a noun, a creature you can count. Flee is a verb, an action. If you could replace the word with run away, you need flee. Note that the past tense of flee is fled, not fleed.
The mix-up shows up when people write about pets or emergencies, for example flee collar instead of flea collar, or residents flea the city instead of flee. Picture the a in flea as an animal and the double e in flee as eek, the cry of someone escaping.