That the symptoms of what are now known to be autoimmune diseases are caused by the presence of an image or representation of elephants and are absolutely nothing to do with an arsehole of an immune system.
(FIL and his sister)
DP didn't have much fun as a child with severe Psoriasis. His father just about got persuaded that UV treatment was OK because 'it's just electric sunshine, MrDPDad'. But other than that, no treatment.
That Psoriatic arthritis doesn't exist. Not heard of it before, so it can't be real.
(MIL)
That Psoriasis, in the absence of any malevolent pachydermal imagery (because it was already banned from the house due to the Grandfather's history of magical induction of illness), was caused by the unfortunate six year old getting lost when playing, as the Psoriasis appeared for the first time at an unspecified point in the year following those five minutes before he worked out he was just the other side of a hydrangea and found his way back to the right garden.
(FIL, MIL and DP's Auntie)
That the BCG contained diseased badgers. (FIL - an OG antivaxxer)
That autoimmune disease simultaneously doesn't exist, as it's all in the mind, and is also real when a doctor says so but is caused by the power of your thoughts because you want to be dependent upon benefits and trap your partner into being a carer out of pure spite. Or your parent wants you to become dependent upon benefits as an independent adult, thus accounting for you having a diagnosis before you're old enough to know what an immune system is. And taking paracetamol means you're a drug addict.
(My ex. It's what his parents taught him, apparently)
That having a diagnosed autoimmune disease affecting your joints means that you can't possibly have another autoimmune disease that's more inconvenient in terms of needing to not have wheat, gluten, barley, oats or milk and you're just a faddy eater who will get over it if every meal for the next 14 years consists of at least two of those things.
(My mother)
That there's no such thing as arthritis in anybody under 70 and anybody claiming to have it is clearly lying to excuse poor attitudes at work and obesity, so must be referred to Occupational Health so they can establish that the employee is lying and can therefore be fired.
(An ex Headteacher. Previously a PE teacher to boot)
That autoimmune diseases are caused by;
Eating nightshade vegetables
Your blood being acidic
Drinking orange juice or having any acidic foods with the exception of apple cider vinegar, which, despite being literally acidic, turns into an alkalising agent once consumed
Eating animal products
Not eating animal products
Not eating enough animal products
Eating too many animal products
Not eating the right type of animal products
Eating the wrong type of vegetables
Eating cooked food
Not eating enough fruit
Not eating nothing but blended fruit
Eating all of the above if not produced in the most expensive manner possible
Vaccinations
Not having a strong enough immune system so that it needs to be 'boosted' by consuming vitamin C in industrial quantities
Constipation
Taking part in exercise or physical activity
Not taking part in exercise or physical activity
The strain of being overweight or obese on the non weightbearing joints even when the person has had a diagnosis since they were a two stone preschooler.
Thinking sad thoughts.
Not having faith in a supernatural being, the exact nature of which can vary
Eating more than once a day
Not fasting regularly
Drinking alcohol
Not drinking alcohol
(Assorted health, wellness and diet gurus that make a shit ton of money from flogging books and media features on their 'magic secret cures' and their worshippers)