According to Burt's rep ... Burt, like everyone else in the country, caught that nasty flu bug that's been going around ... and was coping with the symptoms for several days.
But the symptoms got worse, and Burt was admitted to the hospital. The rep tells us Burt was moved to the ICU earlier today after doctors determined he was "severely dehydrated."
The good news -- we're told doctors are optimistic that he will make a full recovery.
We're told Reynolds' mustache is also doing OK.