Distinguishing Between Growth and Value Stocks Diversify Your Investments

Diversification and Risk

It's important to remember that no matter how diversified your portfolio is, your risk can never be shrunk down to zero. You can reduce risk associated with individual stocks (what academics call unsystematic risk), but there are inherent market risks (systematic risk) that affect nearly every stock. No amount of diversification can prevent that.
Source: http://www.investopedia.com/articles/01/051601.asp