Angie Davidson started to show all the classic symptoms of lupus when she was in her late 30s. These included: ulcere della bocca, depressione, exhaustion, joint and muscle pain, sudorazioni notturne e emicranie. She went from being very fit and healthy to visiting her GP on a weekly basis.

"Despite being only 38, I felt like an old woman. I was exhausted all the time yet couldn't sleep and had severe joint pains. I suddenly became very depressed, which was extremely unlike me as I'm usually a very upbeat person. And I mean serious depression with thoughts of ending it all. I also had terrible problems with my memory. I was constantly going into a room and not having a clue why I was there. Every week, another symptom seemed to show up and I couldn't find out what was going on and doctors kept dismissing my fears as being in my head."

Unfortunately, none of Davidson's symptoms were linked and each one was treated individually. But she was insistent that something wasn't right. "It was very frustrating as I couldn't seem to convince doctors that something was wrong, although in my heart I knew I had a problem. Also, typical of lupus patients, I looked really well," she says.

So, what exactly is lupus?

Lupus, also known as systemic lupus erythematosus (SLE), is an autoimmune disease, where the body's immune system mistakenly attacks healthy tissues and organs. According to Paul Howard, deputy CEO of Lupus UK: "As a result of the excess antibodies in the blood, lupus causes inflammation and damage in the joints, tissues and other organs."

"Normally our immune system protects us from infections. However in lupus, our immune system mistakenly starts attacking various parts of our body, resulting in illness and damage to healthy organs," says GP Dr Mousumi Banerjee.

The exact cause of lupus is not known. "Unfortunately, we still don't fully understand what causes the immune system to misbehave and begin attacking healthy cells in people with lupus, although there are a number of triggers that have been identified," says Howard.

"Lupus is much more common in certain groups; around nine out of 10 cases are in women which suggests a possible genetic and/or hormonal influence. It's also more common in people of Black African, Caribbean and Asian heritage, again indicating possible genetic links. Triggers for the disease include puberty, pregnancy, exposure to UV and certain viruses," he says.

Angie waited four years for her diagnosis. "It was by pure chance that I was referred to a rheumatologist. After asking some questions, he immediately said I think I know what this might be. He took my history and blood tests, and soon after he told me I had lupus. I said: 'Oh thank God. What's lupus?' I was relieved to have found out what was wrong as I could now deal with it. A lot of patients have told me they felt relief too, especially the ones who are misdiagnosed for years," she reveals.

Lupus patients experience an average delay of 6.4 years between developing initial symptoms and reaching a diagnosis. Lupus is often misdiagnosed because of its many different symptoms.

Howard explains: "It can mimic other, more common conditions - due to the range of symptoms lupus can cause, including extreme fatigue, joint and muscle pains, sun-induced rashes, hair loss and kidney problems. It some cases it may look like another disease and be misdiagnosed - approximately 48% of people with lupus are initially misdiagnosed with another condition, such as artrite reumatoide oppure rosacea."

There's no cure for lupus at present but careful monitoring of the condition and a treatment programme enables the condition to be controlled. "The outlook for people with lupus has improved significantly from decades ago as our understanding of the disease has improved," says Howard.

"New treatments have become available and we have learned how to use older medications more effectively. Thanks to these advancements most people with lupus will be able to have a normal (or near-normal) life expectancy. It is rare now, but sadly in some cases lupus can still be fatal," he says.

Davidson controls her lupus with diet, exercise and some lifestyle changes. "I'm very lucky in the fact lupus hadn't managed to do serious damage, such as attacking my internal organs, so I had the choice as to whether to take medication or not; many patients don't have that option. It's a lot of effort, but I have a personal preference to stay off any medications for anything unless it's absolutely necessary," she says.

"Stress is a huge trigger, so I have had to learn not to sweat the small stuff. I moved from central London to a quiet house in Kent and that has made a huge difference. Lupus patients do have a habit of pushing themselves too far (me included) and we can almost certainly expect payback in the form of a flare afterwards, so we do have to learn to pace ourselves a bit."

"Many people may not need treatment apart from using sunscreen and analgesics like ibuprofen," says Banerjee. "Eating a balanced diet, regular exercise and avoiding direct sunlight are beneficial. As symptoms can vary over time, patients need to regularly visit their GP for check-ups. Their GP may refer them to specialists, who may start treatment with steroid tablets or injections. In serious cases, newer treatments with medications that modulate the immune system may become necessary," she explains.