Virologists like simplification of systems

Virologists like simplification of systems. When we want to study the viral entry, we make viruses that perform only the entry. When we want to study only the viral replication, we make viruses that perform only the RNA synthesis. Such method is most useful for the study of dangerous viruses such as hemorrhagic fever viruses. The natural Ebola virus is too danger to study. But we can make the Ebola virus that can perform only RNA replication. In this system, the virus has no dangerousness because it cannot form the infectious viral particles, which means that the virus cannot be propagated. The simplification of systems gives us a safety of the virus research.