In the case of candida, there are 2 situation I can think of as far as my experience tells me.
1 Pathogenic kind which reproduces quickly and has exceptional adhesion properties. This is dangerous and would result in acute illness and high mortality in weak individuals.
2. Opportunistic as a result of other chronic health related problem, genetic problem (TLR or HLA)and or microbiota imbalances not exclusive to the gut.