The vast majority of patients are correctly diagnosed by medical professionals – but some conditions have symptoms that mimic other health problems, and that's when diagnosis can be much more challenging.
Here are six of the illnesses that can be harder to spot in initial medical consultations:
A 2011 Rarer Cancers Foundation study found family doctors were failing to diagnose one in four cancers – although misdiagnosed cancers often include the more common types like breast, colorectal, lung and pancreatic cancer.
They may be misdiagnosed because symptoms mimic other, non-cancerous conditions - inflammatory breast cancer, for example, has similar symptoms to mastitis; colorectal cancer may be mistaken for irritable bowel syndrome (IBS) or ulcerative colitis; lung cancer symptoms can be similar to those caused by pneumonia, bronchitis, and tuberculosis; and pancreatic cancer can be confused with irritable bowel diseases, pancreatitis, and diabetes.
The symptoms of this degenerative disorder of the central nervous system include tremors, stiff muscles and problems with balance or walking. It can be mistaken for Alzheimer's disease, stroke, stress, or a head injury, as there are no lab tests that can diagnose Parkinson's.
An inability to digest gluten which can start suddenly, this digestive disorder can cause abdominal pain, headaches, joint pain, itchy skin, diarrhoea, vomiting and weight loss, although sometimes symptoms can be mild. While a blood test can diagnose coeliac disease, it's often mistaken for other conditions including irritable bowel syndrome (IBS), diverticulitis, intestinal infections, and chronic fatigue syndrome.
Fatigue, rashes and joint pain are some of the common symptoms of this autoimmune disease that can affect almost any part of the body, including, in severe cases, organs like the heart and kidneys. Although blood and urine tests can indicate lupus, such tests are not always conclusive and it's sometimes misdiagnosed as arthritis or fibromyalgia.
Around 3% of Britons have an underactive thyroid (hypothyroidism), which can be diagnosed with a blood test and treated with a synthetic hormone.
But it can remain undiagnosed for years because symptoms can be very subtle, such as tiredness, weight changes, forgetfulness and lack of concentration. Sufferers may be told there's nothing wrong, or that they're depressed.
The progressive autoimmune disease multiple sclerosis (MS) attacks the central nervous system and causes symptoms including muscle spasms, coordination and balance problems, blurred vision, numbness and tingling, and cognitive impairment.
No one test can diagnose it, and it can sometimes be misdiagnosed as a viral infection, lupus, Alzheimer's, or bipolar disorder. Blood tests can rule out some other disorders, and tests including an MRI scan may indicate MS, but can't give a conclusive diagnosis.