Vogue thats the scary part, they genuinely don't know what causes it.
For every scientist/researcher/psychiatrist that thinks its genetic, another will argue its environmental/learned.
Me personally, i think it is genetic. My older sister is equally as severely ill with OCD as i am, and about the same things, toilets, bodily waste germs etc. Yet our parents were not germophobes, they didn't use antibacterial stuff, didn't clean excessively, just normal regular household. Yes, they raised us to wash hands after the toilet, but just a regular quick soap and water wash. Not after coming in from outdoors, never used to wash before eating or anything like that.
My sister knows what her trigger was, a video shown at school about the importance of washing after the toilet and a visual of those incubated gel things that grow all the bacteria cultures. My trigger isn't totally clear as i was only 5 and not at the point of development to remember things long term. From what my parents say i just one day started complaining i didn't feel "finished" after the toilet, like i wasnt fully empty and if i got off i might leak the rest. Sent to a doctor, no UTI, hospital, scans showed no twisted bowel or any deformity to indicate a physical cause, so i was just sent home to deal with it, and clearly it escalated to consume my life.
My dad gets major anxiety to the point of needing to be medicated, either with diazepam or self medicate with alcohol, when bad weather or storms mean high winds, so its probable he has it, but it only affects him in very specific and infrequent situations. His brother has it quite badly, will NOT eat beef, checks all doors are locked multiple times, and those are just the ones i know if (really not a close family). his daughter when she was younger displayed issues with using toilets, but thankfully she seemed to grow out of it. His aunt also claimed to have OCD and be a washer in bleach, but i only met her twice and she was labeled a hypocondriac by most of the family so i have no idea if it was true or not.
Me and my sister can't even agree, as she thinks it was probably learned behavours. I now i did "learn" my germ phobias fro her, it used to just be a fear of toilets for me, not the germs associated with wetting myself or anything. But she used to force me to wash my hands a certain way, never touch waistbands of bottoms or the hems of tops, can't ever touch your underwear, you MUST wash your hands if you even brush against any of them, must wash your hands in a specific pattern ritualised way after the toilet, has to be antibacterial soap etc. And so i became obsessed with germs. I honestly can't tell you why, im not affraid of them making me sick, and if i do get sick, i trust my immune system to fight it, and if that fails, i have total trust in medicine, im not scared of dying, i just genuinely cannot stand the thought of germs being on things, not that theres any consequence to it if there are. It just has to be prevented at all costs.