> Scientists believe the Pasteurella bacteria generally lives harmlessly in the tonsils of some, if not all, of the antelopes. An unusual temperature increase to 37C, however, caused the bacteria to pass into the bloodstream, where it caused septicaemia

"... except because it will be efficiently removed by its immune system (that knows perfectly what is a Pasteurella)" is the part of the phrase that is missing here.

It this does not happen... the question is why this year is not happening.