Do I need an antivirus for a MacBook Air? If so which one?

Do I need an antivirus for a MacBook Air? 

The answer to that question is no but you can only recover from the effects of having installed and actually used the scam "cleaning" product by completely erasing the Mac.

Please read Effective defenses against malware and other threats - Apple Community.

Erase the Mac and reconfigure it from the ground up.

Rule 1 of Macs is don't install junk.

No need to apologize; I realize "erase and reconfigure" is a very brief way of describing how to recover from using "MacKeeper" or similarly categorized destructive products.

Recovery procedures can vary, but the following describes your options in detail. You will need to decide which procedure works within the time and effort you wish to devote to the task, including the possibility of using Time Machine to restore the Mac to a known, operational state, prior to the effects of having used those destructive products. Measures short of completely erasing the Mac are certainly possible, but they will be iterative (trial and error) and can be excessively time-consuming with no guarantee of success.

As a category, non-Apple "cleaning" products are scams. Merely uninstalling them, even when performed in strict accordance with their instructions (which often don't work) is usually an insufficient remedy due to the effects of having used them.

Very briefly stated, the recovery procedure from having used such things is as follows, and in the following order:

1. Back up the Mac using Time Machine or an equivalent backup strategy
2. Remove the scam "cleaning" products according to their instructions, and evaluate
3. Reinstall macOS using macOS Recovery, and evaluate
4. Reinstall all other potentially affected apps, and evaluate
5. Erase the Mac completely, followed by reinstalling macOS, followed by reinstalling essential apps from their original sources, followed by migrating User content from the Time Machine backup.

Since the results of Steps 2-4 are almost always unsatisfactory, I omit them as unnecessarily time-consuming. Step 5 is the nuclear option. An expanded procedure for it is provided below. Be sure to read everything and understand its implications. Less drastic solutions certainly exist, but this one will probably result in the least amount of your time and need for constant interaction. It is also unequivocally supported by Apple's published documentation, is what a Genius Bar would do if you were to pay them to do it, and it is the exact same procedure I use given a Mac that has been affected by similarly categorized products.

To recover from the effects of having used "MacKeeper" to modify your Mac, the additional software you require and the essential files you need, follow the applicable recovery procedure below:

• If you have a backup that you created prior to using MacKeeper, now is the time to use it. For Time Machine, boot macOS Recovery, and at the Mac OS X / macOS Utilities screen, choose Restore from Time Machine Backup. Choose a date preceding the installation of MacKeeper.
• If you do not have a backup that predates the use of MacKeeper, create one now. To do that read Back up your Mac with Time Machine.
  • The recovery procedure will require that you completely erase the Mac using macOS Recovery, and then create a new User Account whose contents will be empty. You will then be able to use Migration Assistant to migrate your essential documents including photos, music, work products and other essential files.
• To erase and install OS X / macOS read How to reinstall macOS.
  • Follow the instructions in that document under If you want to erase your startup disk, in Erase and reformat a storage device in Disk Utility on Mac.
  • Then, follow the procedure in Transfer to a new Mac with Migration Assistant.
  • When asked how you want to transfer your information, select Transfer from a Mac, Time Machine backup, or startup disk.
  • Under Select the Information to Transfer, select only your previous User account and do not select "Applications", "Computer and Network Settings" or "Other files and folders". De-select those choices.
  • Subsequent to using Setup Assistant, you will need to reinstall the essential software you may require, once again remembering to install software only from their original sources, and omitting all non-essential software.
  • "Non-essential software" is a broad category that includes but is not limited to third party "cleaning", "maintenance", and "anti-virus" products.

You have already seen how these apps can affect your computer's operations in a not good way. Apple's OS has its own built-in antivirus and its OS is locked so it can't be changed. An anti virus app simply takes your money for nothing in return. Remove it with the vendor's uninstaller app. Viruses do affect PC's, not Macs.

I understand. That is the reason that Mac must be erased and reconfigured from the ground up.

Merely removing scam "cleaning" products, even when performed in strict accordance with their uninstallation instructions, does not undo the damage they are capable of inflicting.

In fact the uninstallation instructions for "MacKeeper" product happen to be a model of uninstallation perfection. It works with nearly complete effectiveness, but removing it according to those instructions does not reverse its damage. Nothing short of erasing the Mac can accomplish that.

John Galt wrote:

5. Erase the Mac completely, followed by reinstalling macOS, followed by reinstalling essential apps from their original sources, followed by migrating User content from the Time Machine backup.

…

Step 5 is the nuclear option.

I would suggest that before doing this, the OP should have at least two backups of their user data – and that it might be good to back up that data in two different ways.

The idea is that you do not want to have all of the user data in one basket. Backups themselves can fail, and if there is only a single backup, and you wipe the original data, THAT is the moment that Murphy's Law says that your single backup is most likely to fail.

Backups themselves can fail, and if there is only a single backup, and you wipe the original data, THAT is the moment that Murphy's Law says that your single backup is most likely to fail.

👍

"Two is one and one is none."

No, you don't; instead, avoid downloading software from sources other than the Mac App Store and the developer websites.

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