Signing off the post with a tribute to her friend, Gomez said she felt “incredibly blessed”.

“And finally, there aren’t words to describe how I can possibly thank my beautiful friend Francia Raisa,” she added. “She gave me the ultimate gift and sacrifice by donating her kidney to me. I am incredibly blessed. I love you so much sis.”

The singer also highlighted the need for more awareness around the illness, describing it as “misunderstood”, as many people still feel confused about what lupus is, and what causes it.

To help build this awareness, we’ve answered some of the most commonly asked questions about the condition.

What is lupus disease?

Lupus, also known as systemic lupus erythematosus (SLE), is a long term condition causing inflammation to the joints, skin and other organs. Its wide-spread effects are one of the reasons it’s not commonly understood as it affects all sufferers differently.

Symptoms of lupus

In its mildest form, lupus will provoke joint and skin inflammation, as well as symptoms of fatigue. A tell-tale sign of lupus is lupus rash, which takes the form of a red, butterfly-shaped skin rash across the center of the face. It usually covers both cheeks and grazes the bridge of the nose. General inflammation of the skin can also mean flaky, red spots or a scaly, purple rash on various parts of the body, including the face, neck, and arms.

If the disease progresses further or becomes more aggressive, the inflammation may affect organs such as the lungs, heart and kidneys. In severe cases, lupus can cause life-threatening damage to the heart, lungs, brain or kidneys.

Typically, lupus symptoms alternate between flaring up and becoming worse for periods of time and then settling down and going into remission for a few weeks. It is not known why this happens – and some people don’t feel a fluctuation at all.

What causes lupus?

Lupus is an autoimmune disease that causes the body’s immune system to attack itself. Although there are supposed triggers, no one knows exactly what causes it.

It is not contagious, but it’s thought that a viral infection, strong medication, sunlight, puberty, childbirth and the menopause are all possible triggers of the disease.

Although lupus can affect anyone, more women than men get the condition and it’s more common in those with black or Asian heritage.

What is lupus and what causes it? Selena Gomez talks about her battle with the disease