No one really knows the cause of rosacea but there are many theories. The current most popular one is that rosacea is a disorder of the innate immune system. For over twenty years the vascular theory was held as the most promising. What is interesting is at the heart of the innate immune system theory is that this includes an overproduction of cathelicidin (a killer of microscopic organisms that is at the cellular level found in white blood cells and also found in cells on the skin) that is transported in the vascular system, so we have come full circle. There are many other theories worth considering but a good place to start your research is our post on Theories Revisited.