To add on to reasons for susceptiblity to morgellons symptoms, most of which could also apply to susceptibility for any disease:
Heart not as strong as it should be. It may not exactly be overt heart disease problems, but the heart may have been damaged subtlely from childhood illnesses, especially if antibiotics were employed (this is something I learned by observation in traditional Chinese medicine clinic--subtle heart damage was detected in some children I saw being treated by a TCM master. They had taken antibiotics Their tongue had a geographic coating, which according to the TCM master indicated some heart damage.)
Endocrine system is not as it should be. Most common endocrine dysfunctions are thyroid disesase and adrenal problems. Both really do a number on the immune system function. There are other many other endocrine dysfunctions, such as parathyroid.
Internal body temperature is too low. Doctors accept low body temperatures as normal, but if body temperature is consistently below 98.2, guess what? Your immune system is probably not working as it should. Discussion about low body temperatures here:
www.wilsonsthyroidsyndrome.com
Not all low body temperatures are thyroid in origin of course. Low adrenal also can lead to low body temperatures.
Heavy metal toxicity. Big problem for all as heavy metals are pervasive in the environment (lead in bones is much higer today than it used to be). Especially a problem if there are mercury fillings. Heavy metals can also lower body temperature
Genetic defects. Many may have them, but may not even know what that they do especially if they are minor problems, but could make one susceptible to getting sick more easily.
Excessive stress: death of a spouse/child, caregiver for a very sick person for a prolonged period of time, divorce, moving etc).
In the blood type diet book it is stated that type A blood people have low stomach acid. So that, if true, this would make one more susceptible to catching diseases, especially parasites (which I believe morgellons may be).