“Julia isn’t sick” is the same as “Julia is not ill”. It is a natural way of saying the same thing.
बीमार an adjective that means sick/ill. So, you can just say ' जूलिया बीमार है' to say that 'Julia is sick'.
However, बुखार- fever is a noun which means you have to indicate possession. This is usually done with के पास for material possessions but for something like a fever, you can simply use को.