Q:

If a checkbox isn't checked, do something

Having some trouble figuring this one out. If both the checkbox in the second and third td is unchecked, then disable the one in the fourth.
$('#allergies tbody tr').each(function () {
  if ($('td:eq(1) input[type="checkbox"]:not(:checked)', $(this)).length > 0 
      && $('td:eq(2) input[type="checkbox"]:not(:checked)', $(this)).length > 0) {
      $("td:eq(3) input", $(this)).attr('disabled', false);
  }
});

A:

$('#allergies tbody tr').each(function () {
  if ($('td:eq(1) input[type="checkbox"]:not(:checked)').length > 0 
      && $('td:eq(2) input[type="checkbox"]:not(:checked)').length > 0) {
      $("td:eq(3) input").prop('disabled', true);
  }
});

to the specific ones in the tr use
$('#allergies tbody tr').each(function () {
  if ($('td:eq(1) input[type="checkbox"]:not(:checked)', $(this)).length > 0 
      && $('td:eq(2) input[type="checkbox"]:not(:checked)', $(this)).length > 0) {
      $("td:eq(3) input", $(this)).prop('disabled', true);
  }
});

