बीमार bimar بیمار is a persian loan word combines from two words mar and bi mar means snake and bi means without, and it goes back to the time which rats could be the reason of lots of illness and snakes were the enemy of rats which the places had snakes had less rats and more health
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.