Example take Chagas Disease caused by the vampire bug the Vinchuca of which I have seen some so big in the Amazon basin (almost literally) that ten hitched in line could pull a plow. Never the less the Vinchuca is an insect bulbous with blood that has many enemies that want to make it a morsel. When it invades peoples dwellings incorporating thatch or grass or other vegetation building materials to construct with such as used in packed roofs for insulation it can move quickly and hide in these or make its dwelling in these - where it can not be sniffed out so to speak. In Bolivia right now year 2011 thirteen of every one hundred deaths are Chagas disease of the bite of the Vinchuca vampire beetle related. However if Bolivia has a Department like Sacatepequez in Guatemala where the famous city of Antigua, third oldest in the Americas is located, which has a little more money and has a very agressive Chagas control program, then the low Chagas rate figures of that Bolivia Department would be "skewed". It would not reflect the overall Bolivia rate of Bolivia Departments that do not have an agressive Chagas disease control program. In Sacatepequez Department Guatemala if you pack your roof with vegetable material for insulation you will wake up in the morning with the rain in your face and a crew removing your roof. There are Guatemala Departments where this agressive Chagas control program is not in place and in these Guatemala Departments there are Chagas infections. But not in Sacatepequez. Reiterating a Bolivia Department with this agressive Chagas control program would have a low rate of Chagas infection in the Bolivia Departments population and that low rate percentage would not accurately reflect the overall rate of Chagas infection in Bolivia - which is much higher.                                                                               The United States south of the Mason Dixon line used to have a lot of Chagas as it is south of the snow line where north of the Vinchuca can not survive. Probably one in thirteen deaths Chagas related 200 years ago. But none in the southern U.S. year 2011 are so poor as to rely on thatch and grass for building. Thus the bulbous with blood Vinchuca has no escape and refuge from many enemies who want to make it a morsel and it stays away year 2011 from peoples homes. The United States has very few Chagas infections year 2011. There are limits to the altitude the Vinchuca can survive at. The City of El Alto Bolivia at 13,300 feet altitude is a city where the Vinchuca does not survive. It is at the same altitude as Tiwanaku Bolivia about 30 miles distant. In El Alto the dogs I have viewed including the old dogs are very spry. But just a few thousand feet lower the Chagas is very noticeable in many dogs all ages. The reason the Vinchuca must ofter rely on its wings to drop on its prey undetected. But its wings are ineffective at the Al Alto thin air altitude and it wakes its prey up in dropping. A piper or cessna bumps down the runway at 13,300 feet unable to lift off.