Recently I have received emails from different providers and it had a link inside.
I already know it's not the provider emailing me, it's a computer VIRUS they caught.
Once you click on a strange link in an email, these virus are written to jump to your contacts and send an email to everyone with the same link again.

OK, I suck at computer stuff and had that happen to me once and I have to say it was very embarrassing and I had a shitload of explaining to do to the people in my email system. In my case it sent out viagra(or something like that), from what I was told, to everyone.  My question is, how could you tell that it was "bad"?  Anymore I just delete emails from unknowns  and just don't click on a link if that is all that is in the email I from even someone I know.

Often the tone.  "HEY, CHECK THIS OUT" from somebody you rarely email.  Basically, get to the point where you're thinking about this and you'll always get a spidey-sense feel for the spam.

Deleting the unknown emails is good but when you get them from people you know it's harder. But just tame the curiosity and dont click the links, funny thing is most of the emails arrive with no text in the body, just the link.
The links will access you contacts and worse of all , it can keylog everything.
Keylogging would be the most devastating. .

The gals I know would never send such a stupid thing, so I know their computers got infected.

Best thing to do is to contact them and let them know so that they can run a virus scan and get rid of it.

Yup, the tagline & usually no message body(except the link) are the key points to knowing it's BS. And if it happens to you, one usually has no idea, until someone they know tells them, or they get an e-mail back from a contact. It happened to my personal e-mail this winter, & my aunt e-mailed back saying, "Wow, that sounds like a great opportunity, I'm definitely going to check it out!"... huh? I went into sent, & there it was.. so I contacted everyone else, told them don't open it, & told her to run a virus check on her computer ASAP, as I did not send it. If it happens to you, change your password immediately.  

And there are worse things than a virus. I will not open ANY attachment, unless I really trust the person and/or know that the person has a legit reason to send me something.