Whether induced by some alien spore, or purposely caused by medical equipment so they can better treat the patients, the Coma is always just as risky condition as it is in the present day.
Although doctors and machines can do all they can to repair a body, the real battle to live, die, or to remain comatose is waged within the patients mind.
While in the coma, the battle to live or die is waged with the patient reliving old memories, some good, and some bad, so that the ultimate outcome is dependent on (a) the good memories outweighing the bad; or (b) the patient reliving a sequence of their lives similar to the one that put them into the coma and remembering how they won over the odds in the past.
The sequence of these memories can be totally random. Most times, however, they are inspired by what people in the room with their bodies are saying. Other times the doctors can stimulate specific memories electronically to assist the patient with their internal struggle.
[plot device used in the Simpsons, Conan TV show, ST:TNG, Starship Troopers CGI cartoon, who knows what else.]
Usually they are genetically engineered, sometimes as a weapon, some times as a harmless aid that went wrong. [Sliders, the Stand, The Omega Man. ] Other times it seems that the plague evolved naturally. [John Christopher's Empty World. ] . Some times these plagues are unleashed by alien invaders to lessen resistance on a planet. [Dalek Invasion of Earth, and Blood Heat ] These plagues are 100% contagious, and 99% fatal, leaving a scant few survivors to pick up the pieces and restart civilization.
The survivors of these plagues can seem almost random, but normally include one of the people who were working on the disease when it first broke out, a few random adults who seem to have an immunity that leaves them unaffected, and to a greater degree young teens and pre teens who have a strong enough immune system to recover from the disease, or are able to fight the disease off longer than adults.
These plagues, frequently have similar symptoms. (it is important to note, that this is frequently, many times the plagues have totally different symptoms, or some of the symptoms listed.)
Stage 1: Light sensitivity. The infected person feels the need to wear sunglasses and complains that the light hurts their eyes in a very dim room. Light causes the infected person intense physical pain. The level of pain they feel can be used to trace the progress of the disease.
Stage 2: Skin Pigment. During this stage the infected person looses all skin pigmentation. They feel the need to avoid light at all costs. They also undergo personality shifts, causing them to seek out other infected peoples and band together in packs. They also become intensely violent towards people who are not infected, or are still in stage 1 of the disease.
Stage 3: Death. The infected person dies painfully, sometimes the disease stops here, but some of the nastier variations do not.
Stage 4: Total Absorption. The body of the infected person is completely dissolved/eaten by the disease leaving nothing behind but clothing and dust.
These plagues do not seem to effect plants or other animals at all.