Varnado felt nauseous and was first taken to a local hospital in Starkville before being moved to the Northeast Mississippi Regional Medical Center in Tupelo for tests.
Though the illness is not considered serious, spokesman Gregg Ellis said the school is taking "every precautionary measure" with the Southeastern Conference tournament MVP.
"It's nothing serious," Mississippi State coach Rick Stansbury said late Monday night as he left the medical center -- the same hospital Stansbury was in last season when he had a virus. "He had a little breathing difficulty and got dizzy and that's why he went to the hospital. They're going to keep him overnight for observation but there's no reason why he won't be released tomorrow [Tuesday]. There's absolutely nothing that they saw wrong."
Ellis said doctors are checking for everything from a bad diet to exhaustion.
Varnado is one of this year's premier returning seniors after two years as the nation's leading shot blocker.
The 6-foot-9 Brownsville, Tenn., native had 170 blocks last season and 327 in the last two. He needs 141 to tie the NCAA career mark. He also led the Bulldogs with 12.9 points and 8.8 rebounds per game.
He briefly entered his name in the NBA draft last spring before deciding to return to school. He also was a member of the U.S. team that finished third in the World University Games over the summer.
Information from ESPN.com's Andy Katz and The Associated Press was used in this report.