A．The virus is infected into their bodies.
B．The bite of the infected animals passes the virus presenting the saliva to others.
C．The virus has established in their central nervous systems for several years.
D．The virus infect them by some infectious diseases.

A．asking for industrial psychologists’ help
B．studying the effects of fatigue on workers to determine the length of working time
C．establishing a more direct communication between employees and management
D．changing the routine motions of the work itself, changing or moving the tools, improving the working conditions