What happened to my flying squirrel?

Well I had originally had 2 flying squirrels named Mac and Cheese and one day Cheese started having seizures ant then 2 weeks later on easter, his back legs went limp and I thought they were paralyzed so I started looking it up online but nothing came up. At 10:08 P.M., he coughed up blood and died in my arms. I want to know what happened and how to get rid of it if you catch it early because now Mac's back legs are paralyzed. he hasn't had a seizure yet but I can still fix it because i caught it early. PLEASE REPLY!!!!