Focos

04:34 Jul 31, 2016 

I agree with you. The terms "surto" and "focos" in Portuguese aren't used interchangeably as far as I can tell from the few sources I have checked, which supports your choice of the term "focus" in this example.

I have found two definitions of focus.

1- Natural focus of an infection: an ecosystem in which an infectious agent normally persists in nature; for example, yellow fever virus in a jungle monkey-Haemagogus mosquito ecosystem.

2- Focus: The center, or the starting point, of a disease process

I've also found more examples of "focus" being used in this sense. Focus and outbreak aren't used interchangeably here either.

"The initial focus of cases in southeast Texas
suggests that infection spread from Louisiana, probably
via infected birds. Adjusting for reports of WNV
disease in wildbirds, based on the assumption that
WNVinfection in wildbird populations is a precursor to
an outbreak in local equine populations, the equine
epidemic of WNV disease in Texas during 2002
appeared focused within districts in the high plains of
northwest Texas and in northern Texas. Whether WNV
spread to northwest Texas from the focus in southeast
Texas, or from neighboring states or other ..

11:24 Jul 30, 2016 

A focus of cases is a pocket of cases in a confined geographic area.