Aggressive Scaredy Cat

Of my three cats I have one, Nia, who is pretty much scared of everything she is not familiar with and will run and hide in a full blown panic, sometimes to the point where she could hurt herself scrambling to hide. My cats are indoor 100% of the time but recently a new cat has started coming around the windows of the apartment and the first time Nia saw the cat she let out an extremely loud aggressive yowl that startled me and sent one of my other cats hiding under the bed. She sat in the window hissing and making the distressed sound cats will make when introduced to a new cat. My other two cats watched the outsider with interest but there was no vocal or overt physical reaction.

Nia's reaction surprised me considering she goes into panic mode with everything unfamiliar. She was the last of my three cats adopted into the family so she was passive and wanted to hide when introduced to my other two cats when adopted. She has not gone through a situation of having another cat introduced to the house when she is now a member of the family and an outsider shows up.

I am just curious if this is a normal reaction since the outsider cat is more or less an equal to her and she goes into a protective mode verse a person entering the home who she runs away from.

Thanks

Pretty much normal.

Thanks for the reply.

A separate but related question, now that temperatures are coming down and I can open windows to let in the fresh air is there any concern about having cats coming around outside the window that might have health issues when my cats like to sit in the window? Do that need to have physical contact to catch a virus or disease or can face to face contact through the screens be a problem?

Thanks again.