Sporadic

Sporadic (Gr., 'scattered') is a term applied to any disease that is commonly epidemic or contagious, when it attacks only a few persons in a district and does not spread in its ordinary manner. The conditions which determine the occurrence of epidemic or contagious diseases in a sporadic form are unknown. Amongst the diseases which occur in this form may be especially mentioned cholera, dysentery, measles, scarlatina, and smallpox.