Someone can get a lawsuit against you started without you being served.
However for the actual lawsuit to go through and for yourself to get sued in court you have to be served officially and the papers have to be served to you directly and not mailed to you etc.
A process server or Sheriff will have to show up at your door and serve you with papers.
If you are served and do not show up at court then the person suing you will win the lawsuit by default so if you do get sued then you should show up in court to possibly win the lawsuit.
By law it's not legal for you to be sued without you being served.
So if you were sued without you being served then that is illegal.