Ebola and West Nile are foreign. Foreign viruses are incurable in the US. Therefore that is one cause of death by virus
Another reason viruses are deadly is because of the method of attacking a victim. When Marburg virus attacks a victim, the victim incurs symptoms which lead to death
Strength of a virus can make it deadly as well. When the rabies virus is inside a body, it finds its way to the brain, killing numerous organs in the process, which can cause failure
Composition of one's body can be the difference between life and death. Viruses that are innocuous to one person is not to another, like sex transmitted viruses. This also leads to deaths