It can be impossible to tell when another person is suffering from inflammatory bowel disease (IBD), due to the fact that symptoms are largely internal.
But that doesn't mean IBD conditions, such as Crohn’s disease and ulcerative colitis, are easy to deal with.
To raise awareness about the hidden difficulties of living with IBD, Tiffany Jane Anderson, who suffers with Crohn’s disease, posted a "before and after" picture on Facebook showing how she feels on a "good day" compared to a "bad day".
Anderson's image has been liked more than 7,000 times since she posted it on Wednesday and has inspired other IBD sufferers to share their own "before and after" images.
"It took a lot for me to post this picture but I just wanted to raise some awareness for a chronic illness called IBD," Anderson wrote alongside the image, posted on the Crohn’s and Colitis UK Facebook page.
"It can be invisible and I can look normal but still be in agony on the inside. Today I'm having a bad time with all the things that come with IBD.
"People are quick to judge me but you all forget you don't know my story - so please think before you speak."
According to the NHS, the most common IBD symptoms include stomach pain, recurring or bloody diarrhoea, weight loss and extreme tiredness - meaning they're largely hidden from others.
Anderson's post received thousands of shares and likes overnight, and other people suffering from IBD have shared their own "before and after" images on the thread of comments.
Many people have also commented on Anderson's original post to thank her for raising awareness of IBD.
"Thank you for sharing, what you've said is so true, I have ulcerative colitis and most days look fine outwardly but most days the opposite is true, lots of pain and tiredness and worrying if there's a toilet nearby if you go out and that's only part of it, I also have a stoma which brings it's own set of problems," one person wrote.
"People think there's nothing wrong and I know there are a lot of hidden illnesses, so please let's stop judging each other because none of us known what others are going through."