On July 13, the Tanzanian government's chief medical officer Aifello Sichalwe announced that a team of medical experts has been dispatched to the country’s southeastern region of Lindi to investigate an as-yet unidentified disease that has infected 13 people and killed three. Although Tanzania has never officially recorded cases of either Ebola or Marburg, the people afflicted with the deadly disease—which has symptoms including fever, headaches, fatigue, and nosebleeds—were tested for both viruses, the New Zealand Herald reports, but the results were negative.

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According to the Herald, Tanzanian President Samia Suluhu Hassan says that the mysterious disease may be the result of more frequent interactions between humans and animals as the region’s population expands into previously undeveloped areas. While the cause of the disease is unknown, Hassan’s hypothesis lines up with a World Health Organization’s (WHO) statement released yesterday (July 14) that reports a 63 percent jump over the past decade in the frequency of diseases spreading from animals to humans.

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“We need all hands on deck to prevent and control zoonotic diseases such as Ebola, monkeypox, and even other coronaviruses,” Matshidiso Moeti, the WHO’s regional director for Africa, says in the statement.

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For now, officials say the Tanzanian government is working to identify the source of the current outbreak. “The government formed a team of professionals who are still investigating this unknown disease,” Sichalwe says, according to the Herald.