Sarcoidosis is a disease involving abnormal collections of inflammatory cells that form lumps known as granulomas. The disease usually begins in the lungs, skin, or lymph nodes. Less commonly affected are the eyes, liver, heart, and brain. Any organ, however, can be affected. The signs and symptoms depend on the organ involved. Often there are no, or only mild, symptoms. When it affects the lungs there may be wheezing, cough, shortness of breath, or chest pain. Some may have Löfgren syndrome in which there is fever, large lymph nodes, arthritis, and a rash known as erythema nodosum.
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The exact cause of sarcoidosis is not known. The current working hypothesis is, in genetically susceptible individuals, sarcoidosis is caused through alteration to the immune response after exposure to an environmental, occupational, or infectious agent. "

Source : Wikipedia

The disease was first described (and named) in 1887, and since then has been the subject of many research projects aimed at pinpointing its cause(s).

The current view is that sarcoidosis could be caused through malfunction of the immune response after exposure to an environmental, occupational, or infectious agent - in genetically susceptible individuals.

It usually resolves after a period of a few months, with or without drug treatment.

Further information US NIH National Heart, Lung and Blood Institute.