Often when my sister and I visited our grandmother as children, we found her rubbing pungent ointments into her swollen wrists and fingers for her “rheumatism.” We took this for granted. “Grandmothers have arthritis,” we used to think.

Now we know that a balance between genetic and environmental factors play a role in a person’s susceptibility to arthritis, and although you can’t do much about the genes, you can take steps to control the environmental factors.

Years later, when I was beginning my training as a rheumatology fellow, red spots appeared on my grandmother’s legs and arms after she used moth spray on some carpets. I knew this petechial rash – skin spots that don’t fade when you press them – can accompany autoimmune disease, but a thorough work-up revealed no clear diagnosis. Months passed before the next symptoms appeared, without warning and with a vengeance. A spinal artery stroke left her quadriplegic. Her doctors diagnosed lupus and prescribed high-dose corticosteroids, but she spent her last months bedridden and in pain.

Ten years later, my mother developed high fever and enlarged lymph nodes after a flood damaged Montreal’s inner suburbs, where she lived. Her doctors decided she might have been exposed to a virus. The fever resolved, but then returned with severe joint pains and swelling, and her blood tested positive for rheumatoid arthritis, or RA. My mother, then 70, had never had arthritis symptoms before. Doctors also found a breast lump, and in the end, it was cancer that took her, but arthritis symptoms waxed and waned the rest of her life.

Even with this family history, I was surprised when I developed arthritis symptoms – swelling and pain in my knees, elbows, shoulders and wrists. The doctors diagnosed non-specific inflammatory arthritis – not RA and not lupus.

In any “complex” disease, the tendency to develop inflammation comes from genes, but whether it’s arthritis, multiple sclerosis or something else depends on such environmental factors as bacteria, viruses, chemicals or foreign proteins. In addition, there is a “dose effect” of genes – if you inherit many autoimmune genes, you will be more likely to get an autoimmune disease, regardless of environmental exposures. If you inherit very few, then environmental factors – from chemical exposure to major stress – become more important. The genes load the gun and the environment pulls the trigger.