Not neccessarily true. The 'Cahokia Indians' were not the most advanced tribe in North America... I think you have the tribal names confused with a tribe whose name we do not know, who built the mounds nnow known as Cahokia Mounds, across the river from St. Louis, Missouri. The area is an Illinois state park today. Cahokia Mounds wasn't named after the people who built the mounds... the site was named after a subgroup of the Illiniwek people that was known as the Cahokias, who lived at the site long after the original Indians who built it had abandoned the place, and who had absolutely no relation to the builders of the mounds.
The other error is that the moundbuilders did not die out from a disease introduced to the New World from the old... as a matter of fact, we don't know if the 'died out' at all, since there is nothing in the archeological record to indicate they experienced a catastrophic disease or even catastrophic warfare. It may be that their culture changed and they dispersed and merged into other peoples. If disease played a role, it wasn't likely from Columbus or later Europeans, since they were gone from their city long before Columbus. Their civilization had been in decline well before, the mound city of Cahokia itself was virtually abandoned by 1300 AD. We call this early culture the 'Mississippians,' not the Cahokias.
The tribe called Cahokias, as I said, were a much more recent tribe who didn't know who had built the mounds before they arrived in the area. The Cahokias were far less advanced than the moundbuilders were culturally; they lived in houses that were totally unlike those of the long-goone moundbuilders, their agricultural practices were not as advanced as the moundbuilders, their artifacts and religion were wholly dissimilar from those of the moundbuilders. the Cahokias were part of the Illiniwek, othoerwise known as the Illinois Nation, once a proud people, and after Pontiac's War the last of the Illinois were pursued and trapped on the Illinois bluffs known as Starved Rock by the Peoria Tribe, and there they were practically exterminated.
As far as mounbuilders go, (they were very advanced but no more so than the Anasazi, Hopi or Navajo) the great city of Cahokia's orignal inhabitants the Mississippians, dissappeared and their city grew over with trees, then was redicovered by later 'Indians' before the Europeans arrived... but a few tribes related to the moundbuilders were still extant farther south when early French and Spanish explorers arrived... the Natchez of Mississippi, the Choctaws, and the more distant Caddoan people, among a few others. They were not the same people who built Cahokia, but close in art, architecture, and religion to them. There wasn't a whole lot of moundbuilding going on by that time, though... the mound culture was already well past its prime.