#3: Insidious

Lookups for the word insidious were driven by three very disparate triggers: The word spiked when the horror sequel Insidious 3 was announced, and the film's publicity provoked curiosity about the title itself. Increasingly sophisticated malware attacks were frequently described as insidious this year. And finally, the most poignant use of the word followed the death of Thomas Eric Duncan, the first person diagnosed with Ebola in the U.S. In a statement, the hospital's spokesperson said that "Mr. Duncan succumbed to an insidious disease, Ebola." The medical definition of insidious is very specific:

of a disease: developing so gradually as to be well established before becoming apparent

Insidious comes from a Latin word meaning "ambush."