No matter what material your house is built from, you will always find some metal in the structure. And where there is metal, there is a risk of rust forming. Rust forms when metal is exposed to water and oxygen, and once it starts, it can spread and cause a lot of problems.
Depending on what it is that is rusting, the severity of the problem will be different. If the rebar inside of the cement of your foundation starts rusting, this can lead to cracks in the cement and structural damage. If you have a rusty electrical panel, it could cause a short and even lead to a fire starting. These are very serious problems, and they need to be addressed by a professional as soon as possible.
On the other hand, if you have discoloration of your siding or roof from nails rusting, this is not as urgent, even if it isn’t a pretty sight.
To deal with rust, you first need to remove every trace of it, followed by covering any surface that is now exposed to the elements with a paint that will prevent rust from forming again.