It feels like someone has hacked into my phone. The letters don't work but as i look at it someone is typing whatever they want. The app don't slide as usual and i cannot write a text message or a whataapp message. I feel i have a virus in my phone.

There are no known virus or hacked for a non jailbroken phone.

If there is a delay in typing see if this link helps http://osxdaily.com/2013/09/23/ios-7-slow-speed-it-up/

You do not have a virus.

Basic troubleshooting form the manual are restart, reset, restore. Try those to improve performance

This happened on a iPad I had. The screen had gone bad and was not responding to touch or was acting like you were touching just in all the wrong places. Took the device to the Apple store and it was replaced under warranty. If you still have warranty left or AppleCare+ take the phone to the Apple store and get it replaced. If the phone is out of warranty then you will have to pay $269.00 to get a replacement device. Resets of the device will probably not work. I tried all that including a total reset from DFU mode. Only solution was to replace the device.

As the others have said, this is definitely not malware of any kind.

First, try resetting your device. Press and hold both the home button and the sleep/wake button for about 10 seconds, until the screen turns off. Ignore the slider that will appear. Release the buttons, then press and hold the sleep/wake button until the Apple logo appears (if necessary). This is the equivalent of a reboot, and should not affect your data or apps.

If that doesn't help, try restoring your device from backups, and if that doesn't help, restore it to factory settings (ie, do not restore a backup). See:

http://support.apple.com/kb/ht1414

If none of that helps, the issue is caused by some kind of hardware problem, and the device will need to be repaired or (more likely) replaced.