The Backstreet Boys have been forced to cancel their promotional tour for their seventh album, This Is Us, after Littrell contracted influenza A, reports People.
The other members of the group, Nick Carter, Howie Dorough and AJ McLean, were also examined, and do not have any symptoms of the disease.
In 1998, the singer, who is now 34, had open heart surgery to repair a heart birth defect.
Littrell is not the only celebrity to have contracted H1N1. The actor who plays Ron in the Harry Potter films, Rupert Grint, contracted the virus, as well as Tom Meighan of Kasabian, Talisa Contostavlos of N-Dubz and musician Jens Lekman.