Uveitis happens when the eye becomes red and swollen (inflamed).

Inflammation is the body's response to illness or infection.

Most cases of uveitis are linked to a problem with the immune system (the body's defence against infection and illness).

Rarely, uveitis may happen without the eye becoming red or swollen.

Uveitis often happens in people who have an autoimmune condition. This is where the immune system mistakenly attacks healthy tissue.

Autoimmune conditions known to cause uveitis include:

• ankylosing spondylitis – a condition where the spine and other areas of the body become inflamed
• reactive arthritis – a condition that causes inflammation in various areas of the body
• conditions that cause bowel inflammation – such as Crohn's disease and ulcerative colitis
• psoriasis – a skin condition
• psoriatic arthritis – a type of arthritis that develops in some people with psoriasis
• multiple sclerosis – a condition mainly affecting the nerves
• Behçet's disease – a rare condition that causes mouth ulcers and genital ulcers
• sarcoidosis – a rare inflammatory condition that affects the lungs, skin and eyes
• juvenile idiopathic arthritis– a type of arthritis that affects children

Uveitis can also be caused by an infection, such as:

• toxoplasmosis – an infection caused by a parasite
• herpes simplex virus – the virus responsible for cold sores
• varicella-zoster virus – the virus that causes chickenpox and shingles
• cytomegalovirus – a common infection that does not usually cause any noticeable symptoms, but can cause sight-threatening uveitis in people with a lowered immune system
• tuberculosis
• HIV and syphilis are rare causes

Uveitis can also be caused by:

• trauma or injury to the eyes, or eye surgery
• some types of cancers, such as Non-Hodgkin lymphoma, although this is a very rare cause of uveitis

Sometimes, a specific cause of uveitis cannot be identified.

Although uveitis is not passed down through families, a gene known as HLA-B27 has been linked to an increased risk of developing uveitis at the front of the eye (anterior uveitis).

About half of all people with anterior uveitis have the HLA-B27 gene. The gene has been found in people with certain autoimmune conditions, including ankylosing spondylitis and ulcerative colitis.