It is absolutely true that you can be infected by a virus and not even know it. Some of the known viruses can remain dormant for a long time. During this phase it is not possible for a human being to know that the virus has already entered the body. It is sometimes also called the gestation period. When the virus comes out of this dormant period, then only it is possible for the human beings to know that they have been attacked by a virus. Sometimes this late discovery can be very fatal and even lead to death. This dormant form of virus is very difficult to initially understand.