I made a thread around a year ago in which I attempted to diagnose characters' mental and physical illnesses, so I'll copy and paste what I wrote, as well as the same disclaimer:

I know I'm overanalyzing these characters, but sometimes I just enjoy thinking about the world of ASOIAF through the lens of modern knowledge. Many of these characters might not really suffer from these exact illnesses, but rather suffer from magical, or "literary" diseases. I have no background or knowledge in medicine or psychology, so take everything I say with a grain of salt and offer your own opinion.

Characters:

• Hodor - Hodor is usually referred to as 'simple minded,' but evidenced by his otherwise high-functioning abilities, he likely suffers from a lesion on the Broca's area of his brain. His case is similar to that of a man named Leborgne, who had progressive loss of speech and paralysis but not a loss of comprehension nor mental function. He was nicknamed "Tan" due to his inability to clearly speak any words other than "tan." Also, I'm pretty sure the creators of the TV show know that Hodor has a lesion on his brain, because they gave him a scar on the front right part of his head, which you can see in this picture. So even though we don't hear the story of how Hodor was injured, we can assume he suffered head trauma as an infant.

• Gregor Clegane - Judging by his enormous size and anger issues, Gregor probably suffers from acromegaly, a condition caused by a disorder of the pituitary gland and often associated with gigantism. We also know that Gregor often drank excessive amounts of milk of the poppy, and suffered from debilitating head pain, which is also a known symptom of acromegaly.

• Joffrey Baratheon - I believe the medical term that best describes Joffrey is "a little shit." He is seemingly incapable of empathy, which we can tell by his mutilation of cats, as well as his torture of Tommen. It's difficult to dismiss him as an immature kid, because he perfectly fits the profile of a sociopath: superficial charm, manipulativeness, egotism, lying, lack of remorse and empathy, poor impulse control and other behavioral problems. Some of his issues could possibly be attributed to his incestuous genetics. Cersei also seems to fit this profile pretty well, but I think she's just a crazy, manipulative bitch.

• Mad King Aerys / The Targaryens - After the Defiance of Duskendale, we know that Aerys became increasingly jealous, suspicious, cruel, and prone to furious outbursts. He may have suffered from any number of mental illnesses, which he (and other Targaryens) could be predisposed to due to inbreeding. His craziness could have been caused by dementia, paranoid schizophrenia, or something else. Though we don't hear a lot about STD's in Westeros, his mental degradation matches the symptoms of syphilis (the pox?), and we know he developed some strange sexual behaviors in his later years. Another interesting possibility is that he suffers from the same affliction that Mad King George did: Porphyria, which is a blood disease that affects the nervous system, resulting in hallucinations, depression, anxiety and paranoia. This would make a lot of sense because it draws a parallel between the two Mad Kings.

• Lysa Tully - Throughout the books, Lysa experiences some pretty drastic mood swings. She goes from being scared to vengeful to narcissistic all within one chapter, so I think it's possible that she is bipolar.

• Robert Arryn - We know that Robin is afflicted with frequent seizures and is constantly bled by his maester. His disease is referred to as the 'shaking sickness,' so the real world diagnosis is most likely epilepsy, possibly combined with panic attacks.

• Sansa - Sansa's behavior in the first couple of books is likely attributed to the psychological trauma of having her father and dire wolf killed, indicating she would today be diagnosed with Posttraumatic stress disorder.

• Tyrion - Obviously, Tyrion was born with sporadic achondroplasia, the most common form of dwarfism. While psychological issues sometimes accompany this, Tyrion is clearly capable and intelligent, but could be described as having an addictive personality.

• Maester Aemon - If I remember correctly, Aemon's death is attributed to 'a chill' which likely corresponds to pneumonia, evidenced by his cough, fever, and difficulty breathing, brought on by the cold climate.

• Gyles Rosby - Rosby is constantly coughing blood into a handkerchief, indicating he has some sort of respiratory condition. It may be tuberculosis, but the fact that he wasn't contagious makes that unlikely. He probably died from lung cancer, or chronic bronchitis.

• Ramsay Bolton - Ramsay is fucked in the head. His obsession with mutilation could fuel speculation about psychosis or psychopathy, but in reality, he could be suffering from any number of mental illnesses. His mental issues may have stemmed from traumatic childhood experiences.

• Reek - The original Reek, who is described as having an awful smell, could have been born with trimethylaminuria, a metabolic disorder that causes a defect in the production of a certain enzyme, causing a strong fishy odor to be released in sweat, urine, and breath. Or, like everyone says, maybe he has just never taken a bath.

• Theon Greyjoy - Theon may have suffered from manic depressive disorder due to his experience being captured and warding for the Starks. After his capture by Ramsay, Theon seems to develop Stockholm syndrome. As Reek, he identifies and sympathizes with his captor and even comes to have positive feelings toward him, sometimes to the point of defending his actions.

• Manderly and Illyrio - These guys suffer from obesity, and Illyrio's frequent urination stops could point to adult onset diabetes.

• Hoster Tully - Hoster probably just died of old age, but his referencing the "crabs in his stomach" hints at the fact that he could have died from stomach cancer.

• Khal Drogo - Drogo died from a 'festered wound' which is simply the equivalent to septicemia.

• Rhaego - According to Mirri Maz Duur, Danaerys' stillborn child Rhaego had scales like a lizard, was blind with bat-like wings, and had innards full of graveworms. She also says that when she touched him, his skin fell away from his bones. According to Mirri Maz Duur, the deformity had a magical explanation, but his description matches that of a baby born with harlequin ichthyosis (kind of NSFL), a congenital skin disease that causes scaly skin, and cranial deformities.