INSIDIOUS - [adjective] not easily noticed, but harmful, causing bad results
Examples of use
The onset of the disease was very insidious. It looked like a cold at the beginning, but ended up killing thousands of people.
The weather in March is insidious. Deceived by sunny days, people start wearing light clothes too early and get cold.
Learn more English words
English speaking practise
Remember that if you don't use that word in English conversations or writing you'll forget it in a short time. So the best thing you can do to learn this word is just trying to use it in speaking or writing. Remember also that practise is the key to fluent english speaking.
You can always find a partner, who would also like to improve his/her