Often times, dreaming of illness has been shown to be a warning of an impending illness, the nature of which is sometimes revealed in the other symbols in the dream.
If the dreamer has been feeling weak, or under a lot of stress, she should consult a doctor.
If the dreamer has not been feeling ill, this is a symptom of something else in your life being out of kilter.
Again, look to the other symbols in the dream.
Dreaming of others being ill is a warning of upcoming difficulties with your relationships.
Astrological parallels: Pisces, Neptune Tarot parallel: The Moon