As everything else in human body, nothing is so simple.
Please look up the info about Blastocystis Hominis (protozoa)infestation. This bug is causing very nasty symptoms, similar to Candida. One of the side effect is rapid weight loss. Most of the labs can not or don't look for it. After numerous negative test result the poor patient end up ridiculed by everybody including family members. Believe me not a nice feeling. Why I mention this?
This bug is feeds also on friendly bacteria. My condition turn very bad after I started to eating kefir.
I still wonder if I got this bug from a kefir of was already in me. As I said nothing is simple.