Warning! The following contains spoilers for the Doctor Who episode "Demons of the Punjab**.**" Read at your own risk!
Doctor Who's "Demons of the Punjab" was a rather tragic tale, though perhaps not nearly as tragic as what some Doctor Who fans are thinking. The episode contained a certain pep talk by one of the Doctor's companions that has many worried about the future. Specifically, it's being theorized that Graham's harboring a certain secret just as big as Yasmin's nan's hidden history: his cancer has returned.
Some fans' ears perked up during Graham's conversation with Yasmin, who was still reeling from the fact that her grandmother was set to wed a man who wasn't the grandfather she knew. Her confusion and devastation led Graham to suggest there was a reason her grandmother kept her in the dark about it. He went on and essentially suggested the best thing for her to do was "live in the moment" and "figure the rest out later."
Was the advice a product of Graham's age-old wisdom, or because the hidden-information situation hit him a little close to home? Some Doctor Who viewers believed Graham was speaking to his own experiences, and that he may be hiding a saddening cancer re-diagnosis from Ryan and the rest of the TARDIS crew. It certainly fits within the context of the advice he gave, and his general attitude, but does that mean it's correct?
To backpedal just a bit, in case some Doctor Who fans may have forgotten: Graham met Ryan's grandma because she was a chemo nurse when he was undergoing treatment for cancer. The two formed a relationship, and when Graham's cancer had entered remission, they were married, making him Ryan's step-grandfather. Ryan and Graham haven't gotten along so well, although they've obviously bonded some, thanks to the adventures with the Doctor and their shared grief over the grandmother's recent passing.
Going back to the speculation about this potential shocker regarding Graham, cancer is certainly a nasty illness, and one can easily assume it wouldn't have been incorporated in Graham's backstory if it wasn't going to be used in a future plot line. Plus, what ailing person with a limited timeline wouldn't want to join the Doctor on the trip of a lifetime before meeting their maker?
Of course, it's always easy to twist a fan theory and make it believable, even when there are other explanations for things. For example, Graham has already explained he's joined the team because he doesn't want to grieve at home alone with constant reminders to his wife all around. Additionally, we've been given no plainly stated reasons to suspect Graham's cancer has returned. And if it has, he certainly isn't showing signs of illness or fatigue so far.