First, we need to understand what influenza B is. Influenza B, also known as influenza type B, is a type of influenza caused by the influenza B virus. Currently, it is classified as a class C infectious disease, which means it is indeed contagious. The main sources of transmission for influenza B are patients and asymptomatic carriers. Generally, the virus is contagious from one to seven days after onset of the illness, with the highest contagiousness occurring within the first two to three days. Influenza B is a respiratory tract infectious disease, mainly spread through the air and droplets. Most people are susceptible to influenza B.