The gymnast has been put in isolation at his hotel and pulled out of final Friday rehearsals today because of the illness.
A source told Digital Spy: "Louis is the latest celebrity to get this Strictly sickness bug! He is in isolation at his hotel.
"Erin Boag and It Takes Two host Zoe Ball had it last week. Louis hasn't been able to come to the final rehearsals with his pro partner Flavia Cacace today."
Writing on Twitter about his state of health, Smith said yesterday: "The start of a long night #reallyunwell"
He added later: "I hate being sick :(
"Drinking dioralyte is like drinking pool water yuck. But big thanks to @bbcstrictly for getting me some xx."
Smith and Cacace have been near the top of the Strictly leaderboard every week so far and several bookmakers have him as the favourite to win the series.
Strictly continues this weekend at 6.30pm on BBC One.
Photo gallery - Strictly show 5: