The human settlements have been located on the border of the alturas. The climate is tropical. The population in Itenez and Mamorb Provinces is 10,000 inhabitants. The principal activity is cattle raising and the harvest of corn, rice, yucca, bananas, cultivated in chacos on the alturas. The prevailing vegetation consists of open grassland and savanna interspersed with islands of climax and semideciduous forest.
Savannas occupy higher sites, namely the alturas, and are maintained by annual burning.
The farms are usually located on the periphery of the fore islands overlooking the grass covered bajios, and in this ecological nest little villages and isolated settlements have been established where the epidemics of haemorrhagic fever have been observed. Calomys Callosus is a pastoral species occurring in scrub forest when the canopy is sufficiently open to permit the growth of grass. Ectoparasites collected from wild and domestic mammals and from humans were species grouped and inoculated in suckling hamsters and mice. No machupo virus was isolated from 803 pools. Three of the most common species of bats, Myotis nigricans , Molossus major and Eumops sp . were negative for virus isolation.