My guess is that each insurance co has their own specific definition of "pre-existing condition" but the general idea is any disease you have been diagnosed with by a qualified medical doctor (MD, DO or ND) within a certain time frame which has not been "cured" would be considered pre-existing. So examples would be things like Asthma, Fibromyalgia, Chronic Fatigue Syndrome, Osteoarthritis, Lupus, etc. - basically anything that would be considered "chronic" because regular medicine does not know how to "cure" these conditions.
Chronic Lyme Disease, Bartonella (clinical dx only), Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication/Chemical Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, chronic Lymphopenia, etc.; G-Tube; TPN via PICC (plan to start weaning off again soon).
Meds: IV Cipro, Heparin (to flush PICC line), Singulair, Claritin, Domperidone, Colloidal Silver (topically & nasally), probiotics, digestive enzymes, Milk Thistle, Magnesium, homeopathy.