What is the difference between "desde hace" and "desde"?

"Desde hace" is used when talking about an interval of time, a duration, i;e: "Estoy enferma desde hace une semana." ("I have been sick for a week").

"Desde" is used when talking about a specific point in time, i.e: "Estoy enferma desde el lunes." ("I have been sick since monday.").