The route west was arduous and fraught with many dangers, but the number of deaths on the trail is not known with any precision; there are only wildly varying estimates. Estimating is difficult because of the common practice of burying people in unmarked graves that were intentionally disguised to avoid their being dug up by animals or natives. Graves were often put in the middle of a trail and then run over by the livestock to make them difficult to find. Disease was the main killer of trail travelers; cholera killed up to 3 percent of all travelers in the epidemic years from 1849 to 1855.