There are established epigenetic and environmental risk factors for RA. Smoking is an established risk factor for RA in Caucasian populations, increasing the risk three times compared to non-smokers, particularly in men, heavy smokers, and those who are rheumatoid factor positive. Modest alcohol consumption may be protective.

Silica exposure has been linked to RA.

RA reduces lifespan on average from three to twelve years. According to the UK's National Rheumatoid Arthritis Society, Young age at onset, long disease duration, the concurrent presence of other health problems (called co-morbidity), and characteristics of severe RA—such as poor functional ability or overall health status, a lot of joint damage on x-rays, the need for hospitalisation or involvement of organs other than the joints—have been shown to associate with higher mortality". Positive responses to treatment may indicate a better prognosis. A 2005 study by the Mayo Clinic noted that RA sufferers suffer a doubled risk of heart disease, independent of other risk factors such as diabetes, alcohol abuse, and elevated cholesterol, blood pressure and body mass index. The mechanism by which RA causes this increased risk remains unknown; the presence of chronic inflammation has been proposed as a contributing factor. It is possible that the use of new biologic drug therapies extend the lifespan of people with RA and reduce the risk and progression of atherosclerosis. This is based on cohort and registry studies, and still remains hypothetical. It is still uncertain whether biologics improve vascular function in RA or not. There was an increase in total cholesterol and HDLc levels and no improvement of the atherogenic index.

JIA occurs in both sexes, but like other rheumatological diseases, is more common in females. Symptoms onset is frequently dependent on the subtype of JIA and is from the preschool years to the early teenaged years.

The cause of JIA remains a mystery. However, the disorder is autoimmune — meaning that the body's own immune system starts to attack and destroy cells and tissues (particularly in the joints) for no apparent reason. The immune system is thought to be provoked by changes in the environment, in combination with mutations in many associated genes and/or other causes of differential expression of genes. Experimental studies have shown that certain mutated viruses may be able to trigger JIA. The disease appears to be more common in girls, and the disease is most common in Caucasians.

Associated factors that may worsen or have been linked to rheumatoid arthritis include:

- Genetic predisposition; When one family member has been diagnosed with rheumatoid arthritis or another autoimmune disorder, the chances are higher that other family members or siblings may also develop arthritis.

- Females are more likely to develop rheumatoid arthritis than males at all ages.

- A strong belief is held that psychological stress may worsen the symptoms of rheumatoid arthritis. However, when the emotional stress is under control, the arthritis symptoms do not always disappear, suggesting that the association is not straightforward.

- Though no distinct immune factor has been isolated as a cause of arthritis, some experts believe that the triggering factor may be something like a virus which then disappears from the body after permanent damage is done.

- Because rheumatoid arthritis is more common in women, perhaps sex hormones may play a role in causing or modulating arthritis. Unfortunately, neither sex hormone deficiency nor replacement has been shown to improve or worsen arthritis.

The cause of JIA, as the word "idiopathic" suggests, is unknown and an area of active research. Current understanding of JIA suggests that it arises in a genetically susceptible individual due to environmental factors.

The worldwide prevalence of inflammatory arthritis is approximately 3%. Rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, and undifferentiated spondyloarthritis are the most common subtypes of inflammatory arthritis. The diseases occur most commonly in the 30-40 age group.

There are several diseases where joint pain is primary, and is considered the main feature. Generally when a person has "arthritis" it means that they have one of these diseases, which include:

- Osteoarthritis

- Rheumatoid arthritis

- Gout and pseudo-gout

- Septic arthritis

- Ankylosing spondylitis

- Juvenile idiopathic arthritis

- Still's disease

Joint pain can also be a symptom of other diseases. In this case, the arthritis is considered to be secondary to the main disease; these include:

- Psoriasis (Psoriatic arthritis)

- Reactive arthritis

- Ehlers-Danlos Syndrome

- Haemochromatosis

- Hepatitis

- Lyme disease

- Sjogren's disease

- Hashimoto's thyroiditis

- Celiac disease

- Non-celiac gluten sensitivity

- Inflammatory bowel disease (including Crohn's disease and ulcerative colitis)

- Henoch–Schönlein purpura

- Hyperimmunoglobulinemia D with recurrent fever

- Sarcoidosis

- Whipple's disease

- TNF receptor associated periodic syndrome

- Granulomatosis with polyangiitis (and many other vasculitis syndromes)

- Familial Mediterranean fever

- Systemic lupus erythematosus

An "undifferentiated arthritis" is an arthritis that does not fit into well-known clinical disease categories, possibly being an early stage of a definite rheumatic disease.

Because women may be underdiagnosed, the exact incidence of reactive arthritis is difficult to estimate. A few studies have been completed, though. In Norway between 1988 and 1990, the incidence was 563.3 cases per 100,000 for chlamydia-induced reactive arthritis and 5 cases per 100,000 for that induced by enteric bacteria. In 1978 in Finland, the annual incidence was found to be 5835.7 per 100,000.

Reactive arthritis may be self-limiting, frequently recurring, chronic or progressive. Most patients have severe symptoms lasting a few weeks to six months. 15 to 50 percent of cases involve recurrent bouts of arthritis. Chronic arthritis or sacroiliitis occurs in 15–30 percent of cases. Repeated attacks over many years are common, and patients sometimes end up with chronic and disabling arthritis, heart disease, amyloid deposits, ankylosing spondylitis, immunoglobulin A nephropathy, cardiac conduction abnormalities, or aortitis with aortic regurgitation. However, most people with reactive arthritis can expect to live normal life spans and maintain a near-normal lifestyle with modest adaptations to protect the involved organs.

Seventy percent of people who develop psoriatic arthritis first show signs of psoriasis on the skin, 15 percent develop skin psoriasis and arthritis at the same time, and 15 percent develop skin psoriasis following the onset of psoriatic arthritis.

Psoriatic arthritis can develop in people who have any level severity of psoriatic skin disease, ranging from mild to very severe.

Psoriatic arthritis tends to appear about 10 years after the first signs of psoriasis. For the majority of people, this is between the ages of 30 and 55, but the disease can also affect children. The onset of psoriatic arthritis symptoms before symptoms of skin psoriasis is more common in children than adults.

More than 80% of patients with psoriatic arthritis will have psoriatic nail lesions characterized by nail pitting, separation of the nail from the underlying nail bed, ridging and cracking, or more extremely, loss of the nail itself (onycholysis).

Enthesitis is observed in 30 to 50% of patients and most commonly involves the plantar fascia and Achilles’ tendon, but it may cause pain around the patella, iliac crest, epicondyles,

and supraspinatus insertions

Men and women are equally affected by this condition. Like psoriasis, psoriatic arthritis is more common among Caucasians than African or Asian people.

Inflammatory arthritis can be disabling to the point where people with the diseases can lose their jobs, which can cause psychological distress. Because it is typically progressive, those who lose their jobs are unlikely to re-enter the workforce after leaving due to their diagnosis. Programs now aim to retain those with inflammatory arthritis by preventing work-related injuries and by making necessary accommodations in the workplace. A 2014 Cochrane review found low-quality evidence that work focused interventions, including counseling, education, advocacy, and occupational medicine consultations, were effective in retaining workers with inflammatory arthritis.

Mortality is increased in people with AS and circulatory disease is the most frequent cause of death. AS patients have an increased risk of 60% for cerebrovascular mortality, and an overall increased risk of 50% for vascular mortality. About one third of those with Ankylosing spondylitis have severe disease, which reduces life expectancy.

As increased mortality in ankylosing spondylitis is related to disease severity, factors negatively affecting outcomes include:

- Male sex

- Plus 3 of the following in the first 2 years of disease:

- Erythrocyte sedimentation rate (ESR) >30 mm/h

- Unresponsive to NSAIDs

- Limitation of lumbar spine range of motion

- Sausage-like fingers or toes

- Oligoarthritis

- Onset <16 years old

Arthritis is predominantly a disease of the elderly, but children can also be affected by the disease. More than 70% of individuals in North America affected by arthritis are over the age of 65. Arthritis is more common in women than men at all ages and affects all races, ethnic groups and cultures. In the United States a CDC survey based on data from 2007–2009 showed 22.2% (49.9 million) of adults aged ≥18 years had self-reported doctor-diagnosed arthritis, and 9.4% (21.1 million or 42.4% of those with arthritis) had arthritis-attributable activity limitation (AAAL). With an aging population, this number is expected to increase.

Disability due to musculoskeletal disorders increased by 45% from 1990 to 2010. Of these, osteoarthritis is the fastest increasing major health condition. Among the many reports on the increased prevalence of musculoskeletal conditions, data from Africa are lacking and underestimated. A systematic review assessed the prevalence of arthritis in Africa and included twenty population-based and seven hospital-based studies. The majority of studies, twelve, were from South Africa. Nine studies were well-conducted, eleven studies were of moderate quality, and seven studies were conducted poorly. The results of the systematic review were as follows:

- Rheumatoid arthritis: 0.1% in Algeria (urban setting); 0.6% in Democratic Republic of Congo (urban setting); 2.5% and 0.07% in urban and rural settings in South Africa respectively; 0.3% in Egypt (rural setting), 0.4% in Lesotho (rural setting)

- Osteoarthritis: 55.1% in South Africa (urban setting); ranged from 29.5 to 82.7% in South Africans aged 65 years and older

- Knee osteoarthritis has the highest prevalence from all types of osteoarthritis, with 33.1% in rural South Africa

- Ankylosing spondylitis: 0.1% in South Africa (rural setting)

- Psoriatic arthritis: 4.4% in South Africa (urban setting)

- Gout: 0.7% in South Africa (urban setting)

- Juvenile idiopathic arthritis: 0.3% in Egypt (urban setting)

Between 0.1% and 1.8% of people are affected. The disease is most common in Northern European countries, and seen least in people of Afro-Caribbean descent. Although the ratio of male to female disease is reportedly 3:1, many rheumatologists believe the number of women with AS is underdiagnosed, as most women tend to experience milder cases of the disease. The majority of people with AS, including 95 percent of people of European descent with the disease, express the HLA-B27 antigen and high levels of immunoglobulin A (IgA) in the blood.

Having more than one risk factor greatly increases risk of septic arthritis.

Palindromic rheumatism is a disease of unknown cause. It has been suggested that it is an abortive form of rheumatoid arthritis (RA), since anti-cyclic citrullinated peptide antibodies (anti-CCP) and antikeratin antibodies (AKA) are present in a high proportion of patients, as is the case in rheumatoid arthritis. Unlike RA and some other forms of arthritis, palindromic rheumatism affects men and women equally. Palindromic rheumatism is frequently the presentation for Whipple disease which is caused by the infectious agent "Tropheryma whipplei" (formerly "T. whippelii").

Anti-citrullinated protein antibody is frequently associated.

Worldwide prevalence of spondyloarthropathy is approximately 1.9%.

The exact causes are not yet known, but a number of genetic associations have been identified in a genome-wide association study of psoriasis and psoriatic arthritis including HLA-B27.

Polymyositis, like dermatomyositis, strikes females with greater frequency than males.

Relapsing polychondritis is an autoimmune disease in which the body's immune system begins to attack and destroy the cartilage tissues in the body. It has been postulated that both cell-mediated immunity and humoral immunity are responsible.

Reasons for disease onset are not known, but there is no evidence of a genetic predisposition to developing relapsing polychondritis. However, there are cases where multiple members of the same family have been diagnosed with this illness. Studies indicate that some genetic contribution to susceptibility is likely.

Many individuals have mild symptoms, which recur infrequently, while others may have persistent problems that become debilitating or life-threatening.

Neisseria gonorrhoeaeis a common cause of septic arthritis in sexually active patients under 40 years old. The bacteria is spread through the blood to the joint following sexual transmission. Other symptoms of disseminated gonococcal infection can include tenosynovitis and dermatitis.

Polymyositis is an inflammatory myopathy mediated by cytotoxic T cells with an as yet unknown autoantigen, while dermatomyositis is a humorally mediated angiopathy resulting in myositis and a typical dermatitis.

The cause of polymyositis is unknown and may involve viruses and autoimmune factors. Cancer may trigger polymyositis and dermatomyositis, possibly through an immune reaction against cancer that also attacks a component of muscles.

Increased risk of developing knee and hip osteoarthritis was found in those who:

- work with manual handling (e.g. lifting)

- have physically demanding work

- walk at work

- have climbing tasks at work (e.g. climb stairs or ladders)

Increased risk of developing hip osteoarthritis over time was found among those who work in bent or twisted positions.

Increased risk of knee osteoarthritis was found in those who:

- work in a kneeling or squatting position

- experience heavy lifting in combination with a kneeling or squatting posture

- work standing up

In "gout", the acute inflammatory arthritis is caused by excess uric acid caused by either overproduction or under-excretion. Before the age of menopause, women have a lower incidence than males, but the rates are equal above this age. Gout can cause mono- or polyarthritis, but usually results in monoarthritis first.

A number of studies have shown that there is a greater prevalence of the disease among siblings and especially identical twins, indicating a hereditary basis. Although a single factor is not generally sufficient to cause the disease, about half of the variation in susceptibility has been assigned to genetic factors.

As early human ancestors evolved into bipeds, changes occurred in the pelvis, hip joint and spine which increased the risk of osteoarthritis. Additionally genetic variations that increase the risk were likely not selected against because usually problems only occur after reproductive success.

The development of osteoarthritis is correlated with a history of previous joint injury and with obesity, especially with respect to knees. Since the correlation with obesity has been observed not only for knees but also for non-weight bearing joints and the loss of body fat is more closely related to symptom relief than the loss of body weight, it has been suggested that there may be a metabolic link to body fat as opposed to just mechanical loading.

Changes in sex hormone levels may play a role in the development of osteoarthritis as it is more prevalent among post-menopausal women than among men of the same age. A study of mice found natural female hormones to be protective while injections of the male hormone dihydrotestosterone reduced protection.