It's very unlikely that it would be a virus, they tend to be alot more subtle. Trying to guess isn't so stupid when you think about it, commonly people doing this would have a list of frequently used passwords and a program called a bot which will automatically try to login to a very large number of accounts with passwords from said list, because of the sheer number of accounts it becomes increasingly likely that someone has used a password that is on the list and that their account can be compromised, because of this you should steer clear of using actual words in passwords, again this is unlikely due to systems like captcha, which make it harder for people to automate this, but not impossible.
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