Autorun is beneficial and dangerous for computer users. if computer user are not technical then Autorun is great for that users but How Autorun can be danger?? Actually based malware is very popular and it can be easily install in victim computer .
Autorun is function for windows operating system, due to this option hackers can run malicious software without your permission/notification, I hope you have understood merit and demerit of Autorun. From my suggestion always turn off autorun option because you will not install software everyday. when you need install any types of software then you can install by double click on .exe file (executable file).
USB immunizer is necessary if you insert USB type flash drive or any type of storage device like micro SD card or Pen drive. if you have installed USB immunizer in your computer and someone will insert USB drive then first of all USB immunizer will immunize . this will check every data and will try to find malicious application.
USB immunizer disable autorun related thread and you will feel safe yourself. You can download free USB immunizer which is provide by bitdefender Antivirus >> Download USB Immunizer