Hair loss on cats

I have a siamese mix cat that is approx 12 years old. Until recently she was the picture of health. 2 nights ago, I noticed that she has these patches of hair loss. She is strickly an indoor cat, never even been outside. The patches of hairloss, 2 days ago was a bit smaller that quarter size, today it is closer to a half dollar size and now there are more smaller areas of hair loss appearing. They don't seem to be bothering her. I am just wondering if you can give me any idea as to what it might be. I have 4 other cats and I am concerned it could become contagious. Any info you could give me would be helpful. Thanks.

[Aleda M Cheng, D.V.M., C.V.A]

She really should be evaluated by a vet.  Hair loss can be related to mites, ringworm and other fungal infections (which are contagious), endocrine disorders, problems with the hair shaft and more.