In addition to viral and bacterial disease, I know for sure that stress causes its' own share of illness. Asthma, chronic pain, migraines ... these are all stress related illnesses.
No doubt about it, modern life is stressful. But stress itself? It is invisible. It is as see-through as habits of thought and reaction to events. Stress doesn't show up until a temper snaps, a pencil breaks, a child hits, or a parent yells. So: How to keep the cobwebs from gathering? Spring cleaning, spiritual thought, meditating helps.