Vitality
Typically used with: Endurance.
In short: stay healthy. The Vitality skill is used to keep intrusions upon your health at bay, such as illness and contaminants. This skill essentially represents your ability to flex your immune system and physical composure. If you want to get on all of the carnival rides, even the spinning ones, Vitality is for you.
Healing
A character heals an amount of HP equal to the sum of their Endurance and Vitality for each full night of rest. No roll is required.
Resisting Illness
You can make a Vitality check to ward off illness, the impairment we all experience under detrimental circumstances (e.g. emotion, stress, disease, parasites, diet, drugs, odors). Each cause of illness presents its own DL to resist — the higher the DL, the more unavoidable the illness.
You might need to overcome motion sickness. Someone may have replaced your milkshake with chocolate laxatives. Maybe you're sailing a quarantine ship. If you meet or exceed the DL, you remain healthy. If you fail the check, you contract the illness in question and move one stage down the Knockout Track for each level of severity (see the "Illness" entry in Chapter 9: Adventurers Beware).
A fightable illness allows an unhealthy character to combat the illness at repeated intervals. Each illness featured in this book will list this frequency (e.g. fightable every 12 hours). A successful check releases one stage of the Knockout Track blocked by the illness. A Critical Failure blocks an additional stage of the Knockout Track. Once all of the Knockout Track stages blocked by the illness are released, the creature makes a full recovery.
A limited illness, on the other hand, will go away on its own (assuming you live through it).
Resisting Contaminants
You can also make a Vitality check to repel body-invading substances and organisms. Each contaminant has its own DL — the higher the DL, the more aggressive the intruding agent.
If you meet or exceed the DL, you resist the contaminant and its effects. Otherwise, you experience all the benefits and drawbacks the contaminant has to offer. For example, poisons inflict Continual Damage, pathogens impose illness, and stimulants wake you up. See the "Contaminants" section of Chapter 9: Adventurers Beware for more information.
Resisting Spells
Certain spells call for a Vitality check to resist, for instance Disease, Blood Corruption, and Sicken. In this case, you may add any points in Magic Defense to this roll. The mage makes a Casting Roll. You make a Vitality check. If your result meets or exceeds the Casting Roll, you elude the effects of the spell.