Jill is constantly worried that she will get sick. although she feels fine now and believes that she is healthy, she still worries endlessly about developing a serious illness. most likely jill would be diagnosed with

Jill is suffering from his illness phobia. 

Becuase, Jill is constantly worried that she will get sick. although she feels fine now and believes that she is healthy, she still worries endlessly about developing a serious illness indicates that she is suffering from a phobia.  in fear of illness. She thinks that illness will cause her misery and pain. She imagines her condition of severe and acute illness and gets tense and excessive tension also gives rise to depression and other diseases. Now most likely Jill would be diagnosed with an illness phobia