Some of these viruses have also been reported to cross the species barrier and cause disease or sub-clinical infections in humans.

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Although there's a rough total of reported cases, we don't know how many there have really been, and how often these are serious, because we have little idea how many people have sub-clinical symptoms.

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The disease is so heavily concentrated in the poorest tropical countries, and overwhelmingly in sub-Saharan Africa, that nobody even bothers to keep an accurate count of clinical cases or deaths.

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