YouTube advertisements had hidden viruses that hijacked your computer's CPU and made it secretly mine cryptocurrency. Has your computer been running very slow while you watch YouTube, this might be why.
Link to ARSTechnica's article about it : https://arstechnica.com/information-technology/2018/01/now-even-youtube-serves-ads-with-cpu-draining-cryptocurrency-miners/
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Thanks a lot for these infos. Did not know yet that that also happened also on Youtube.