If YouTube takes files from 3rd parties and simply displays them, then viruses are possible. This is more true of ads placed via ad-broker on other websites. To get ad revenue a webmaster provides a space where the ad is inserted. The ad is provided by a 3rd party who pays the ad broker for placement. Neither the webmaster nor the ad broker have any visibility into the content of the ad, which could even contain code (ads which move or present UI elements have code to make those things work)
If YouTube takes files from 3rd parties and simply displays them, then viruses are possible. This is more true of ads placed via ad-broker on other websites. To get ad revenue a webmaster provides a space where the ad is inserted. The ad is provided by a 3rd party who pays the ad broker for placement. Neither the webmaster nor the ad broker have any visibility into the content of the ad, which could even contain code (ads which move or present UI elements have code to make those things work)