When:
Lookups spiked on October 8, 2014.
Why:
The first person to be diagnosed with Ebola in the United States, Thomas Eric Duncan, died of the disease in Texas. A hospital spokesperson said, "Mr. Duncan succumbed to an insidious disease, Ebola."
When used to describe a disease, insidious means "developing so gradually as to be well established before becoming apparent." In more general contexts, it means "having a gradual and cumulative effect" or "subtle."
It comes from the Latin word meaning "cunning" or "deceitful" and is related the Latin word for "ambush."
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