Does Steam download viruses

As long as you download the official steam at store.steampowered.com you will be alright. Steam is made by Valve, one of the biggest game manufacturers. If they put viruses in a download, that would create trouble no end. In regards to games, it has never happened before, so it is very unlikely to ever happen in the future. Steam probably test all the games submitted to them anyway. But, as always, keep you wits about you when downloading things, even from steam.