Mayo Clinic vaccine researcher Gregory Poland counts more than a dozen new diseases that have emerged in the last few decades: HIV, Lyme disease, West Nile virus, avian influenza.

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The United States is experiencing its biggest spike in West Nile virus since 2004, with 241 cases of the disease reported nationwide this year so far, including four deaths, health officials said last weekend, before the latest totals.

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West Nile virus peaked in 2002 and 2003, when severe cases of the disease reached nearly 3, 000.

BBC: West Nile virus death toll in US jumps by a third

According to the United States' Centers for Disease Control and Prevention (CDCP), it was the worst year for West Nile virus since 2003.

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