How were african rulers unique with their community meetings

True blindness may be caused by trauma to the eye, severe conjunctivitis due to  ibr (infectious bovine rhinoceros) or pink eye or other infections, leptospirosis, lead poisoning, selenium toxicity, fungal infections, vitamin a  deficiencies, rabies, vitamin b12 deficiency, neoplastic of the orbit or  conjunctiva, cataracts, congenital malformations, etc. 

One of the things that allowed native americans in mexico to settle cities larger than those in areas to the north was "b. the development of cities that were built by an earlier civilizations," since mexico had been the home of some of the world's most advanced civilizations before the columbian invasion