What is the symbolism of illness in the novel, All Grown Up?

Illness, in many different forms, serves to symbolize the demons of people. Andrea suffers from mental health issues... something she never really commits to dealing with. With Sigrid's illness, there is also the issue of avoidance, as the dying baby is contagious in a manner, with Andrea not wanting to hold her niece or even visit because she would rather not be a part of that sadness.