Casein is a protein from milk. It's present in yogurt and dairy product. A great proportion of people who have candida are highly sentive to casein. Sometimes way more sensitive to casein than gluten. One teaspoon of yogourt would give me genital burning for days. Who has candida with a leaky gut should never ever eat food with casein.
You only need a small amount to feel bad. What is particular with candida is that one day some food won't make you sick and the other day it will. It depends of several factors like fatigue, toxins build-up, stress, degree of inflammation of your intestinal lining, etc.