Virus scan and Mac Security

Read through a few websites on a good virus scan for Mac and it looks like Bitdefender is a good choice. Before installing it, got a few questions.
1) Is the software only able to scan for viruses? How about malware, adware and spyware etc? If these are not scanned, what would be a good software for those?
2) If there is any virus and malware etc. on the computer before the software is installed, is the software able to take care of it? Does this apply to such apps (virus scans) for the iPhone?
3) If a person is able to get into the wifi network that the computer is currently using, is there a way to detect whether the person is spying or putting viruses onto the computer?
4) A good virus scan is said to be able to scan for more than 99% of the viruses, would it be possible to use two virus scans at the same time?

Thanks.

 

Thanks for the reply. For both the virus and malware scan, does it scan for infections that might be on the computer before such program was installed? Are there any programs for adwares and things that may affect the computer?