How is the cure percentage determined?

When a patient is fully diagnosed and about to be cured, a "cure percentage chance" is displayed. This ranges from 20% up to 99%. Assuming the number is accurate, I want to find out how exactly this number is determined. So I'm interested in the formula, not the general description, that part I know of; since here's some factors that have empirically been verified to affect it:

• Inherent disease difficulty. Grout (the easiest disease) always has a 99% rate, while a very hard disease like Touch of Midas can go down to 20 percent.
• Staff skill level in "cure". This is determined by a combination of Seniority (each point giving a 10% skill bonus) and happiness/energy for another 20% boost for a happy, energized doctor or nurse.
• Machines. Level 2 and 3 machines increase the percentage.
• Room objects such as medicine cabinets.
• A cap at 99%.

Exactly how to take the variables and calculate the percentage is unclear to me.

This is valuable information to figure out if I can say give "Stamina Training" to a doctor instead of another level of "Cure" while still reaching 99%, or how many diagnosis levels I can get away with in the DNA room while still curing a reasonable amount of patients.

For the precise details, I highly recommend the following Steam guide: Illnesses, diagnosis, tips and more under the section "How does treatment work?" I'm sure you'll find the rest of the guide similarly interesting!

In summary, the cure chance is affected by the following:

• Illness difficulty
• Diagnosis certainty
• Machine upgrades, boost objects and staff skill (grouped together)
• Cure cap (usually 99%)

The following formula is given:

CureChance = DiagnosisCertainty * (((100-IllnessDifficulty) * (1-(StaffSkill/100+Upgrades/100) + CureCap * (StaffSkill/100+Upgrades/100)) /100)

You'll also see a subsection titled "Very Important info for staff training" which answers your other question. Since the treatment cap of a doctor/nurse is 100%, you should consider things like stamina training after levels 4-5, whereas for diagnosis staff, you should keep on training them in diagnosis skills.