Flea bites leave you with swollen, itchy marks. Hence the reason why your pets scratch so much after they’ve been bitten. And, in some cases, flea bites cause allergic reactions in humans. But the real problem is fleas carry insect-borne diseases just like mosquitoes and ticks. According to the CDC, the number of illnesses caused by flea bites tripled from 2004 to 2016. So if you’re dealing with a flea infestation, you need to act fast!