You get the virus - simple as that.
Mine froze my PC so I could only use it on 'safe' mode - it is apparently quite difficult to remove - off to get mine extracted today (hate using this ***** laptop).
There are several versions circulating at present - my computer minder tells me that he is sorting out four customers today.
There appears to be a hacking epidemic at present - it took me half-dozen goes to open this forum because of invasive pop-ups - it seems to have become a lucrative big-business.
Ransom-virus is apparently the new kid on the block - pay us xxx and we'll take off what we've just put on.
That's capitalist enterprise for you, I suppose!