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

Most cases of septic arthritis involve only one organism; however, polymicrobial infections can occur, especially after large open injuries to the joint.

- Staphyloccoci

- Staphylococcus aureus - the most common cause in most age groups. Can be caused by skin infection, previously damaged joint, prosthetic joint, or intravenous drug use.

- coagulase-negative staphylococci - usually due to prosthetic joint

- Streptococci - the second most common cause

- Streptococcus pyogenes - a common cause in children under 5

- Streptococcus pneumoniae

- Group B streptococci - a common cause in infants

- Haemophilus influenzae

- Neisseria gonorrhoeae - the most common cause of septic arthritis in young, sexually active adults. Multiple macules or vesicles seen over the trunk are a pathognomonic feature.

- Neisseria meningitidis

- Escherichia coli - in the elderly, IV drug users and the seriously ill

- Pseudomonas aeruginosa - IV drug users or penetrating trauma through the shoe

- M. tuberculosis, Salmonella spp. and Brucella spp. - cause septic spinal arthritis

- Eikenella corrodens - human bites

- Pasteurella multocida, bartonella henselae - animal bites or scratches

- Fungal species - immunocompromised state

- Borrelia burgodorferi - ticks, causes lyme disease

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.

Septic arthritis is due to a bacterial infection to the joint. It requires urgent joint washout in the operating room followed by intravenous antibiotic therapy for large joints. Small joints or children can be treated with repeated aspirations and intravenous antibiotics.

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.

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.

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.

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.

Arthritis mutilans' parent condition psoriatic arthritis leaves people with a mortality risk 60% higher than the general population, with premature death causes mirroring those of the general population, cardiovascular issues being most common. Life expectancy for people with psoriatic arthritis is estimated to be reduced by approximately 3 years.

Synovitis symptoms can be treated with anti-inflammatory drugs such as NSAIDs. An injection of steroids may be done, directly into the affected joint. Specific treatment depends on the underlying cause of the synovitis.

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.

Note that reactive arthritis can also occur secondary to urethral infection. In that case, the term enteropathic arthropathy would not be used.

Arthritis is the most common cause of disability in the USA. More than 20 million individuals with arthritis have severe limitations in function on a daily basis. Absenteeism and frequent visits to the physician are common in individuals who have arthritis. Arthritis can make it very difficult for individuals to be physically active and some become home bound.

It is estimated that the total cost of arthritis cases is close to $100 billion of which almost 50% is from lost earnings. Each year, arthritis results in nearly 1 million hospitalizations and close to 45 million outpatient visits to health care centers.

Decreased mobility, in combination with the above symptoms, can make it difficult for an individual to remain physically active, contributing to an increased risk of obesity, high cholesterol or vulnerability to heart disease. People with arthritis are also at increased risk of depression, which may be a response to numerous factors, including fear of worsening symptoms.

Synovitis is the medical term for inflammation of the synovial membrane. This membrane lines joints that possess cavities, known as synovial joints. The condition is usually painful, particularly when the joint is moved. The joint usually swells due to synovial fluid collection.

Synovitis may occur in association with arthritis as well as lupus, gout, and other conditions. Synovitis is more commonly found in rheumatoid arthritis than in other forms of arthritis, and can thus serve as a distinguishing factor, although it is also present in many joints affected with osteoarthritis. Long term occurrence of synovitis can result in degeneration of the joint.

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.

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.

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)

Worldwide prevalence of spondyloarthropathy is approximately 1.9%.

It is not always certain why arthritis of the knee develops. Most physicians believe that it is a combination of factors that can include muscle weakness, obesity, heredity, joint injury or stress, constant exposure to the cold, and aging. Cartilage in the knee begins to break down and leaves the bones of the knee rubbing against each other as you walk. Persons who work in a place that applies repetitive stress on the knees are at a high risk of developing this condition. Bone deformities increase the risk for osteoarthritis of the knee since the joints are already malformed and may contain defective cartilage. Having gout, rheumatoid arthritis, Paget's disease of bone or septic arthritis can increase your risk of developing osteoarthritis.

Some physicians and most podiatrist believe that Pes Planus (flat feet) cause increased rates and earlier incidence of knee osteoarthritis. In a study of army recruits with moderate to severe flat feet, the results showed that they had almost double the rate of knee arthritis when compared to recruits with normal arches.

25% of cases progress to severe destructive arthritis. In the United States and Canada, mortality is estimated at about 4% and in Europe, mortality is estimated at 21.7%.

Many rheumatic disorders of chronic, intermittent joint pain have historically been caused by infectious diseases. Their etiology was unknown until the 20th century and not treatable, like Lyme disease (in the Northern and Northeastern US), coccidiomycosis or Valley fever (in the Western US), and Chikungunya in India and a myriad of causes for postinfectious arthritis also known as reactive arthritis like, for example, the once very common rheumatic fever after Group A Streptococcus infection up to the rare Whipple's disease.

Major rheumatic disorders currently recognized include

- Back pain

- Bursitis/Tendinitis of the shoulder, wrist, biceps, leg, knee cap (patella), ankle, hip, and Achilles tendon

- Capsulitis

- Neck pain

- Osteoarthritis

- Palindromic rheumatism has been theorized to be a form of rheumatoid arthritis.

Although these disorders probably have little in common in terms of their epidemiology, they do share three characteristics: they cause chronic, often intermittent pain, they are difficult to treat and are collectively very common.

Rheumatic diseases caused by autoimmunity include:

- Ankylosing spondylitis

- relapsing polychondritis

- systemic lupus erythematosus

- rheumatoid arthritis

- gout, inflammatory arthritis, pseudogout

- juvenile arthritis

- Sjögren syndrome

- scleroderma

- Polymyositis

- Dermatomyositis

- Behçet's disease

- Psoriatic arthritis

Arthritis mutilans occurs mainly in people who have pre-existing psoriatic arthritis, but can occur, if less often, in advanced rheumatoid arthritis; it can also occur independently. Psoriasis and psoriatic arthritis are interrelated heritable diseases, occurring with greater heritable frequency than rheumatoid arthritis, primary Sjogren's syndrome and thyroid disease. Psoriasis affects 2–3% of the Caucasian population, and psoriatic arthritis affects up to 30% of those. Arthritis mutilans presents in about 5–16% of psoriatic arthritis cases, involves osteolysis of the DIP and PIP joints, and can include bone edema, bone erosions, and new bone growth. Most often psoratic arthitis is seronegative for rheumatoid factor (occurring in only about 13% of cases), and has genetic risk factor overlap with ankylosing spondylitis with HLA-B27, IL-23R77, and IL-1, however, as of 2016, immunopathogenesis is unclear.

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.

In the US it affects about 250,000-294,000 children and teens making it one of the most common childhood diseases .