Rheumatoid arthritis (RA), a form of inflammatory arthritis and autoimmune disease, occurs in all ethnic groups and all parts of the world. In the United States, rheumatoid arthritis affects an estimated 1.5 million people – almost 1 percent of the adult population.

While no one is immune to RA, some people are more likely than others to have the disease. The greatest risk factor for developing RA is being a woman. Women with RA outnumber men with the disease three to one. In fact, according to the American College of Rheumatology, as many as 3 percent of women may get RA in their lifetime.

Other factors that increase RA risk include the following:

Age. While RA is diagnosed in older teenagers and adults of all ages (younger teens and children may be diagnosed with a related form of inflammatory arthritis called juvenile idiopathic arthritis), it typically begins in women between the ages of 30 and 60; in men it often occurs later in life.

Family history. Having a family member with RA increases the odds of having RA; however, the majority of people with RA have no family history of the disease.

Genes. Research has shown that people with a genetic marker called the HLA shared epitope have five times the risk of developing RA compared to people without the marker. Other genes that up RA risk include signal transducer and activator of transcription 4 (STAT4), a gene that plays an important role in the maturation of immune system cells called T-cells,  and protein tyrosine phosphatase, non-receptor type 22 (PTPN22), a gene involved in controlling the activity of T-cells.  These same genes are involved in other autoimmune diseases in some people, and not everyone with RA has genes associated with its risk.

Smoking. A 2010 Swedish study published in the Annals of the Rheumatic Diseases suggests smoking may be responsible for as many as one in five cases of RA overall and a larger percentage of cases in people whose blood tests positive for anti-citrullinated protein antibodies, an antibody associated with more severe RA. A person’s specific risk of RA varies depending on how much and how long they smoke; however, for heavy smokers, an increased risk persists even 20 years after quitting.

Obesity. A recent study conducted by researchers at Mayo Clinic and published in the journal Arthritis Care and Research found that people who are obese are 25 percent more likely to develop RA than their normal-weight counterparts.