Salt grains

In measles, a highly contagious viral disease spread mainly by airborne respiratory secretions, characteristic lesions may be seen in the mouth on the inner aspects of the cheeks that appear as if there are salt grains splattered against a red background. These lesions are known as Koplik spots, so named in honor of Henry Koplik the New York physician who described them.
Reference:

Steichen O, Dautheville S: Koplik spots in early measles. CMAJ, 2009 180(5):583. Go to reference

Measles virus (electron microscope image)

Image from Wikimedia Commons