This is a very challenging question, because even daring to ask the question can seem immoral, crass, or uncaring, especially when it arises out of medical work. But at some point it becomes essential to ask this question when we strive to understand illness in the context of broader patterns of human growth and development.
The usual, reflexive answer to asking "is it ever good to get sick?" is "No." We perceive illness as painful, dysfunctional, and representing a failure, a breaking down of the machine of the body. Sometimes this failure comes through an invader (like a viral or bacterial infection), sometimes it comes from mistreatment (poor diet, alcohol, drugs), overuse of our body, or overexposure to toxic agents. Sometimes it comes from an unknown failure in our own cellular or genetic machinery. When all illness is characterized in this way--as a failure--we make lists of all possible failures and conclude that avoiding any and all such possible "failures" is good. Working to create less failures then becomes our definition of health. We make checklists of the things we want to stop from happening and plan that, in due course, when more failures can be avoided, everyone will thereby be happier and better. This kind of thinking is very precise and concrete, and on a certain level impossible to argue with.
Such "failure" thinking becomes less satisfying, however, when we stop to look at how, as human beings, we are creatures not only of physical machinery, but also of feeling, sensing, and spiritual activities. Consistent relationships begin to emerge between who we are as individual human beings and how we are in our bodies. There is a mutual interweaving between body and spirit. These relationships are actually recognized more and more in conventional medical studies, though usually without any explanation of how they are linked, and often with fervent attribution to the presence or absence of some specific cellular enzyme or chemical messenger. These studies say things like: when "X" happens, "Y" is more likely, or when "Y" happens, "X" is more likely. The studies show that certain patterns are associated with each other, though they cannot say why. One example includes the finding that people with depression are more likely to develop heart disease, and also (conversely, but independently), that people with heart disease are more likely to suffer from depression (meaning that either illness can be your starting place, though with time it becomes increasingly probable that you will move to the other illness experience as well).[i] Very often, sitting in anthroposophic conversation with people about the "gesture" of a particular physical illness, we discover that there are very similar parallel experiences of soul and spirit. As an example: if we get recurrent cramping pains, tight muscles, and shallow breathing, there is almost always an accompanying emotional or social experience of anxiety and constriction, which may even extend to times when the physical symptoms are no longer present. We can get a cramping that is both physical and emotional. Another example: it is not unusual to find that someone with severe seasonal allergies is often also "open" and sometimes too exposed to the sensory world around them. These links are consistent and true. They create the foundation for much of the anthroposophic medical work. These links could make us fatalistic and make us just worry that because one part of our life is wrong or imbalanced other areas are sure to follow. But what it really, actually means is that there is the possibility for us to work through an illness in one realm, and influence a much larger and more comprehensive process or way of being. The physiologic process of illness and healing can shift a broader overall imbalance.
When we find this interweaving, then sometimes being allowed to work through a physical illness can be very good. Physical illness makes us slow down or stop when we otherwise might not. It is interesting to note how different people are affected by different illnesses to varying degrees. Take an infectious process, like a viral illness that brings with it a tight, irritating cough, and observe in a group of children which ones get the illness and to what degree. For some it will be mild, for others a greater challenge. What is fascinating is that when a child goes through an illness with a tight, irritating cough, and really overcomes it (without the use of a lot of medications that suppress symptoms), the child generally is calmer afterwards. Perhaps before the illness, the child was often irritable or anxious, "wound a little too tight," but now that is gone. Through the process of the illness, the child has worked physiologically through something that was otherwise present more emotionally or psychologically. Similar experiences happen for adults too, as regularly people come to new insights or capacities that would not have been met and transformed had there not been an illness process.
When we take a very long view of human life and biography, it is possible to look and see that some of the most meaningful transformations in outlook or capacity come through wrestling with illness. With a spiritualized view of health and illness, we can then begin to recognize that where there are imbalances, there are also illnesses that offer corresponding doorways to better wholeness. A kind of repertoire of illness processes starts to emerge. This is true for the classic inflammatory illnesses of childhood, it is also true for the more chronic diseases of adult life. Flu and fever can be potent balancers.
So when the opportunity or possibility for an illnesses has been eliminated, checked off the list, it is true that we do not have to worry about that particular disease process. But it is also true that a "doorway" for a physiologic balancing is now gone. If it corresponds to an imbalance that we carry within us, then we will have to find other ways for working with it.
Further sources of information:
"Blessed by Illness" L. F. C. Mees, MD
"The Vaccination Dilemma" Christine Murphy, editor.
[i] Some recent articles that continue to explore this relationship include: "Depressive Symptom Frequency and Prevalence of Cardiovascular Diseases--Analysis of Patients in the National Health and Nutrition Examination Survey," Amer J Ther 2015 Feb. 5; "Revisiting the J shaped curve, exploring the association between cardiovascular risk factors and concurrent depressive symptoms in patients with cardiometabolic disease: Findings from a large cross-sectional study." BMC Cardiovasc Disord, 2014 Oct 28;14:139; "Depression following myocardial infarction--an overseen complication with prognostic importance." Dan Med J. 2013 Aug;60(8):B4689.