Getting Fired while on Doctors Leave ?

I recently received papers in the mail ( today ) that wanted me to elect to take Cobra Insurance. I read further and it said that I was terminated on June 22, 2011. I have been under doctors care due to Severe Vertigo since June 17, 2011 and still have doctor appointments with specialists in another week or so. I faxed several doctors excuses to the company, even telephoned them and let them know I wouldn't be at work due to Vertigo, as was written on my doctors excuse. Is it legal for the company to terminate me while still in treatment ? I am an equipment operator that has alot of peoples lives in my hands the entire duration of work. I operated a boom truck the last day that I worked and it felt like the boomtruck took off drifting on me as I was operating it with people in a manbasket. That is only one instance that I experienced. The vertigo has not gone away after several doses of antibiotics and a CT scan showed that I have Pansinusitis. What are the laws when it comes to termination of an " at-will " employee ? Thanks to all that commented.