PacifiCare to Buy American Medical  CHICAGO (Reuters) - PacifiCare Health Systems Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=PHS.N target=/stocks/quickinfo/fullquote"&gt;PHS.N&lt;/A&gt;  said on Wednesday it agreed to buy American Medical Security  Group Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=AMZ.N target=/stocks/quickinfo/fullquote"&gt;AMZ.N&lt;/A&gt; for \$502 million, in a bid by the health  insurer to broaden into the individual and small-business  markets.