An iceberg disease is a disease which only causes severe and easily identifiable symptoms in some patients. The classic example of an iceberg disease is Celiac disease, a condition associated with an inability to digest gluten. In some cases, symptoms make a Celiac diagnosis relatively straightforward, but in other instances, a patient may experience abstract, intermittent symptoms which make a diagnosis challenging.
This term references the popular idea that only a small part of an iceberg projects from the surface of the water; the same concept is referenced in the saying “the tip of the iceberg.” The idea is that only a fraction of patients with this kind of disease actually seek medical treatment and receive a firm diagnosis, while the vast majority may never receive treatment, often because they don't recognize the fact that their symptoms have a distinct pattern.
For a patient who suffers from an iceberg disease, the disease can be extremely frustrating. As symptoms come and go, the patient may have trouble finding a doctor who takes the situation seriously, and several wrong diagnoses may be arrived at before the doctor and patient finally get to the bottom of the problem. Some patients are accused of being hypochondriacs who are simply faking to get attention, and their concerns may be dismissed.
Iceberg diseases can remain hidden and stealthy for years, sometimes only manifesting in a period of extreme stress, or having such vague symptoms that the patient doesn't notice until the problem becomes severe. This can be problematic in the case of a disease which causes slow degeneration, as patients may not grasp the fact they have a problem until a substantial amount of damage has been done.
While iceberg diseases are certainly a threat, it's no reason to lunge to the doctor's office, unless someone has experienced a consistent set of symptoms or general malaise for an extended period of time. Being able to describe and define the symptoms is very helpful when discussing an ephemeral medical problem with a doctor, and it's also a good idea to be able to link a symptom to a specific incident. For example, someone who experiences vague indigestion and intestinal problems after eating dairy products might have a mild form of lactose intolerance.