Allergy is the great mimic. In some ways the body and immune system are not very clever. It can only react to things in one way – that is with inflammation. Inflammation causes of redness, swelling, pain, heat and loss of function. When one looks at a diseased area, one can see those signs, but it does not tell you what the cause of those signs is. So for example looking at an area of inflammed skin one may not be able to tell if it was infected, allergic, had been sun-burnt or frozen, had acid spilled on it or whatever? For example seeing a person with hay fever you may not be able to distinguish this from a head cold. Hay fever sufferers may get a fever too!
One can be allergic to anything under the sun, including the sun. For practical purposes, allergies are split up into allergies to foods, chemicals (includes drugs) and inhalants (pollens and micro-organisms bacteria, mites, etc)
People with undiagnosed food allergy often initially present with symptoms due to inflammation in the gut (irritable bowel syndrome) and inflammation in the brain (mood swings, depression, brain fog in adults or hyperactivity in children).
However the inflammation can occur anywhere in the body resulting in asthma, rhinitis, eczema, arthritis and muscle pain, cystitis or vaginitis etc or a combination of symptoms.
If the cause is not identified, the inflammation often becomes more generalised resulting in chronic fatigue.