Doctor Who: 9 Ways To Kill The Doctor Permanently
4. The Chen-7 Disease
Part of the fun in travelling with the Doctor is not knowing what to expect. The Doctor rarely conducts thorough background checks of planets before excitedly piloting the TARDIS towards them, and while this spontaneity must be thrilling, it also has the potential to be completely and utterly disastrous for everyone involved.
Case in point: The Girl Who Waited, another Series 6 episode. Here, the Eleventh Doctor takes Amy and Rory to the beautiful planet of Appalapachia, but because he doesn't scope it out beforehand, he doesn't know that the whole place is under lockdown due to an outbreak of Chen-7 - also known as "The One-Day Plague".
It's called this because it kills its victims within a single day, and that includes Time Lords. The Doctor mentions that Chen-7 won't even allow him to regenerate, forcing him to stay inside the TARDIS for the remainder of the episode.
Fortunately, he doesn't get infected, but here's a thought Doctor: maybe do a bit of local research instead of blindly wandering around the universe like a lost dog?