For months, Vicky Veness was misdiagnosed with asthma. Her viral Facebook photo proves that you don't always look sick when you have a life-threatening illness.

By many measures, Vicky Veness is the picture of health. The 30-year-old personal trainer from the UK describes herself as a healthy eater, a runner, and a non-smoker. You wouldn't know anything was wrong from the smiling photo she posted of herself on Facebook last week.

That photo has since gone viral, with more than 2,400 likes and nearly 700 comments—because shortly after she snapped the pic, Veness found out she has stage 4 lung cancer.

"This photo was taken a few hours before I was diagnosed with stage 4 lung cancer," she began the caption to the photo. "When you have cancer, you won't necessarily look ill on the outside. The symptoms might be much more subtle and only show themselves occasionally."

In Veness's case, her symptoms were brushed off by doctors as asthma for 18 months, she told the Daily Mail. Lung cancer is most common among adults ages 60 and older, according to the American Lung Association (ALA), and it's usually (but not always) caused by a history of smoking. So it's understandable why Veness's doctors would have suspected that her symptoms were caused by something less serious, like asthma. Signs of asthma can look very similar to those of lung cancer, including coughing, wheezing, and trouble breathing.

Up to 20% of lung cancers develop in non-smokers, and those lung cancers that aren't tobacco-related tend to strike younger people, especially women, according to the ALA. Exposure to air pollution, asbestos, and radon gas can also cause lung cancer, as can a history of chest radiation, like from treatment from another type of cancer. Some non-smokers may also inherit genetic mutations that lead to lung cancer, as the disease sometimes runs in families.

It's a harrowing diagnosis. Lung cancer is the leading cause of cancer deaths in the U.S. among both men and women. According to the ALA, more than half of people who are diagnosed with lung cancer die from the disease within a year.

But Veness's message is one of empowerment. "The moral of the story is this, if you feel unwell for whatever reason, it doesn't matter how silly you might think it might be, see your doctor, question everything, and keep going back until you get the answers you need."

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