You may have opened an email and thought it was from a friend.
It was spam and then contacted all of your friends and sent them things too.
Try this and see if it stops for you;
To change your password:
1. From the Yahoo! Home page, click on your user name in the upper right.(You will need to be signed in already)
2. From the drop down select Account Info
3. You will be asked to sign in again to enter your Account Info page.
4. Click Change your password
5. You will be taken to a page that will look like the one below.
View Full Image
6. Enter your current password.
7. Enter and confirm your new password.
8. Click Save. You will see a confirmation page when you have successfully changed your password.
9. Click Continue.
1. Sign into your Yahoo! account.
2. Click on your name at the top of the page and select "Account Info" from the pull-down list. You will be required to verify your current password.
3. Under "Sign-In and Security," click "Update password - reset info."
4. You can choose one of the security questions we have made available or your can choose your own.
5. Be sure to click "Save" after you have provided your new information.
1. From the Yahoo! Home Page, click on your Yahoo! ID (You will need to be signed in to proceed).
2. From the pull down menu select Account Info
3. You will be re-directed to a sign in page.
4. Sign in using your Yahoo! ID and password.
5. In your Account Info page you will see a block called Contact Information
6. Click Change next to your Yahoo! Email address.
7. From the Manage your email addresses page you can change, delete, or add a new alternate email address.
8. Click Save
Change your security questions also!
Also; make sure you have a really good Anti virus protection.
I use AVG, it doesn't cost anything if you do their free version and Malwarebytes.
If it is a virus that has you, one of these 2 usually detect it! http://www.malwarebytes.org/
Download the one of your choice (i have both),
Install it and run it.
It will detect if you have rampant viruses.