"Asymptomatic carriers" refers to individuals who are infected with a disease or virus but do not show any symptoms. They can still spread the illness to others without feeling or appearing sick themselves. Full definition
Plowe is trying to develop surveillance methods that hit the «sweet spot,» by which he means sensitive enough to detect very low parasite loads in asymptomatic carriers — microscopy is too crude — but portable enough to use in the field. (sciencemag.org)