I hear ppl getting viruses when they open the e-mail. Is that possible or do you have to download a file (the attachment) that comes with the e-mail? How can one put a virus on e-mail without attachments?

generally no, you have to execute the file first
what email program are you using?

Bugs in the email client can be exploited.

e.g. Buffer overflow causing code hidden in the malicious email to be executed. Microsoft Outlook Express and Microsoft Outlook have a lot of those in the past... but they're fixed now. Though if you install from a really old installer and do not update it, then you're vulnerable.