You won't always know if you have a virus. If it takes much longer for your Mac to wake up than it used to, there may be something lurking in the shadows.
If your Mac just isn't responding the way it should, you might have malware planted into your system somewhere. If this happens, you should immediately change your password to stop any more messages being sent from your account. You may also discover that your personal email or social networking sites are sending out messages that you didn't write. These ads tend to feature adult content or claim that a virus has been detected on your device and will often appear in places that you don't normally see ads. If you start to notice weird advertisements popping up in strange places on popular websites like YouTube, you might be infected.
