Ketosis is a natural process that occurs when fats are converted into energy by the body -- usually when there is not enough glucose (carbohydrates) to provide for the body's energy needs. Instead, the fat is broken down into energy, and "ketone bodies" are the molecular by-products of this metabolic process.
Ketosis may occur during fasting, after an extended period of exercise, or when a high-fat/low carb diet is followed. It can also happen during an illness when nausea and/or vomiting make it difficult to keep food down.
Ketosis in and of itself is not a harmful process and occurs among diabetics and non-diabetics.
However, when ketone levels become excessively high, a condition known as diabetic ketoacidosis, aka DKA, can occur.