
<h3>Description</h3>

<p>The <code>cd4</code> data frame has 20 rows and 2 columns.
</p>
<p>CD4 cells are carried in the blood as part of the human immune system.  One of
the effects of the HIV virus is that these cells die.  The count of CD4 cells is
used in determining the onset of full-blown AIDS in a patient.  In this study of
the effectiveness of a new anti-viral drug on HIV, 20 HIV-positive patients had
their CD4 counts recorded and then were put on a course of treatment with this
drug.  After using 