From @U2
Unfortunately, the rigors of touring and the strain of singing night-after-night took a severe toll on Bono's pipes...especially during the group's monstrous Popmart tour in 1997.
Having picked up the bad habit of inhaling the cheroots he puffed on the Zoo TV tour, Bono became a full-fledged smoker while on the road; a habit which, combined with a severe sinus infection, had a disastrous effect on his voice...which he lost on more than a few tour dates, particularly the band's history-making show in Sarajevo.
Fortunately, after the tour, he was admitted to the hospital to have the problem corrected and has since quit smoking, after doctors told him it would permanently damage his singing.