What causes asthma? This site takes five pages to say, "We're not exactly sure." So why am I giving it 5 stars? Because that's the truth, and because it's a very worthwhile five pages. In medical literature, being able to say "I don't know" in five easy-to-read pages is an almost impossible feat of brevity, and though there aren't many conclusive answers in this article, it's an excellent summary of what IS known.