What makes the picture even more complicated is that people in less developed countries may have developed a resistance to some serious infectious diseases - a resistance that people in advanced countries do not have.  And those people from less developed countries do not show any outward signs that they are carrying the disease in their bodies.  (see article:  "Screening of imported infectious diseases among asymptomatic sub-Saharan African and Latin American immigrants: a public health challenge."  from   NCBI website - National Center for Biotechnology Information).