Fibrosis is a condition present in many chronic disease. Say after a heart attack when the heart tries to repair itself from injury caused by lack of oxygen. What we end up getting it is a scarred fibrotic cardiac tissue (prevelence 7-8%).This impairs function of heart leading heart failure. Then we injury induced by alcohol in liver. When liver scars and becomes hardened we call it liver cirrhosis. Liver cirrhosis due alcohol and viruses combined was 23rd cause of death in 2010 report. The list just goes on. This condition is either cause or consequence of many disorders. So it's pretty common. But there are rare fibrotic diseases too like scleroderma. Which affect like 200/million adults. So answer to your question depends which disease you pick. By TA Wynn's estimate, if you pick a random death, there is 45% chance that there was some of fibrotic tissue involved.