The CAUSE of the flu epidemic of 1918 remains unknown to this day.

Pathology refers to the study of diseases and the changes they cause in the body. While the pathology of the 1918 flu has been studied extensively, understanding the disease's effects on the body does not equate to identifying its underlying cause. Pathology provides insights into the disease's effects but not the initial factors that triggered the epidemic.

Transmission deals with how a disease spreads from one host to another. The 1918 flu epidemic's transmission methods are relatively well understood; it spread through respiratory droplets. However, knowing how the disease spread does not reveal the original cause or factors leading to the outbreak.

Mortality rate refers to the number of deaths in a specific population, often expressed as a proportion. The high mortality rate of the 1918 flu is well-documented, with millions of deaths worldwide. However, mortality rate indicates the epidemic's severity rather than its cause.