After the initial investigations were completed, there was a breakthrough. Lab tests showed that the virus affecting humans was a form of encephalitis (en-seh-fah-LIE-tis). Encephalitis is a disease whose most serious symptom—or sign—is swelling of the brain. This matched the symptoms of patients in the New York hospital. The source of transmission for encephalitis is mosquitoes. This also matched clues that public health workers had found in their investigations.

However, not everyone was convinced that the initial diagnosis was correct. It took collaboration—people working together—among different health fields for the disease to be correctly diagnosed as West Nile virus.

Write down two to three sentences to explain how the doctor at the zoo helped the investigators determine that West Nile virus was the cause of the outbreak.

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