Difference Between $.getJSON() and $.ajax() in jQuery

I am calling an ASP.NET MVC action

public JsonResult GetPatient(string patientID)
{
...

from JavaScript using jQuery. The following call works

$.getJSON(
'/Services/GetPatient',
{ patientID: "1" },
function(jsonData) {
  alert(jsonData);
});

whereas this one does not.

$.ajax({
  type: 'POST',
  url: '/Services/GetPatient',
  data: { patientID: "1" },
  contentType: 'application/json; charset=utf-8',
  dataType: 'json',
  success: function(jsonData) {
    alert(jsonData);
  },
  error: function() {
    alert('Error loading PatientID=' + id);
  }
});

Both reach the action method, but the patientID value is null w/ the $.ajax call. I’d like to use the $.ajax call for some of the advanced callbacks.

Any thoughts appreciated.

Difference Between $.getJSON() and $.ajax() in jQuery