Compare data from two tables and check the same data

Hello I have two tables in my database.

The first one is table disease which look like the above...

And the second is table patient which look like this ...

And I have an edit page where I want to give the user the ability to update his/her diseases. An example is the below..

What I want to do is to check the disease column from the table patient with the column name from the table disease and check that if a data from the table patient is the same with the data from the table disease then check from the checkboxes the same disease.

I tried to find a way to do it by I couldnt Here is my code...

<?php
$sql = "SELECT name FROM disease UNION SELECT disease FROM patient WHERE username='$username'";
$query_resource = mysql_query($sql);

  while( $name = mysql_fetch_assoc($query_resource) ):
?>
    <span><?php echo $name['name']; ?></span>
    <input type="checkbox" name="disease[]" value="<?php echo $name['name']; ?>" /><br />


<?php endwhile; ?>

The way I store the diseases in my database is this...

$disease = implode(",",$_POST["disease"]);

Because a user might have many diseases