A Symptom is a status effect most often caused by contracting a Syndrome. They range from mild temporary annoyances to lifelong disabilities to even death if untreated. An affected dwarf will need to be diagnosed at a Hospital before a treatment can be prescribed.
 List of Known Symptoms
Effects: Areas affected become prone to infection, this becomes more and more likely as severity goes up. While harmless in itself, unless in the lungs in which the unlucky dwarf will be unable to breathe, a resulting infection might cause a loss of life or limb if not properly dealt with.
Effects: Areas affected will continue to bleed for the duration of the effect and will not taper off like a normal wound. In effect it suggests that whatever caused the wound had an anti-coagulant in the venom. This will quickly cause an affected creature to bleed out if the syndrome is not immediately treated.
Effects: Causes pain long after the blunt trauma that caused it is over. Even the most severe bruises will heal without intervention, but may hint at fractures under the skin.
Effects: Dwarf will stop and cough occasionally. While nearly harmless by itself severe coughs will sometimes bring up blood. In this case it's a sign of internal hemorrhaging.
Effects: Creature might stumble/fall in a random direction when attempting to perform an action (including moving [!!!]). The more severe the dizziness the more likely this is to happen. Beware that stumbling, much like dodging, might end up putting a dwarf in a moat, or chasm, or magma, or off a wall, or falling just enough to land unconcious for a second on a cage trap. The list goes on.
Effects: Creature tires faster than normal and requires more sleep. This can be almost unnoticeable if the severity is low, but results in narcoleptic fits if high.
Effects: Appears to reduce the ability to focus. Intense fevers can also cause stunning. While a fever in itself seems relatively harmless, it's often just the most visible symptom. Feverish dwarfs blink with a red "X".
 Impaired Function
Effects: Creature will be unable to use affected body parts for the duration. Much of the functionality here is mirrored in swelling, but this allows certain parts (such as organs) that normally can't swell to still cease functioning
Effects: Creature will stop and Vomit. How often this happens depends on severity. Sustained and intense vomiting will cause dwarfs to dehydrate.
Effects: Creature will not be able to feel affected body parts. This makes pain a non-issue, but also hampers their use (I.E. Something with numb hands will find using weapons difficult, something who has a leg that's fallen asleep with have difficulty standing). Prolonged numbness will lead to permanent sensory nerve damage.
Effects: The affected tissue starts to rot away, which needless to say is a medical emergency. If action isn't quickly taken to remove the tissue (through either surgery or amputation) it will spread throughout the body and kill the creature. Also commonly seen in Undead creatures.
Effects: A sign of infection. Infected areas heal much slower and might kill if especially severe or widespread.
Effects: Subject might give into pain and fall unconscious depending on toughness. The more severe the pain the more likely this can happen, and the more often. This can be very dangerous if the dwarf is in combat.
Effects: The creature can't move. Severity only decreases the chance of resisting the symptom. Untargeted paralysis often leads to suffocation in smaller creatures.
Effects: Impedes the ability to use affected areas. Normally fairly harmless but can cause death by suffocation if it affects the throat. Prolonged swelling will lead to necrosis.
Effects: Affected creature will pass out for the duration of this effect, even if the creature isn't in pain or extremely tired.
 Vomiting Blood
Effects: Much as excessive vomiting can lead to death from dehydration, vomiting blood can lead to death from blood loss. Considered to be a separate condition from coughing blood. If your creature happens to have green blood you'll be unable to tell this from nausea without checking the tiles.