4. Fill in the gaps in these sentences:1)Virology is often considered

1 Virology is often considered as a part of microbiology
2 Viruses can be classified according to the host cell they infect: animal viruses, plant viruses, fungal viruses,and bacteriophages (viruses infecting bacteria, which include the most complex viruses).
3 Another classification uses the geometrical shape of their capsid (often a helix or an icosahedron) or the virus's structure (e.g. presence or absence of a lipidenvelope).
4 A virus is a small infectious agent that can replicate only inside the living cells of organisms.
5 Viruses are found in almost every ecosystem on Earth.
6 Viruses cause a number of diseases in eukaryotes.
7 Viral infections in animals provoke an immune response that usually eliminates the infecting virus.
8 The evolution of viruses, which often occurs in concert with the evolution of their hosts, is studied in the field of viral evolution.
9 While viruses reproduce and evolve, they don't engage in metabolism and depend on a host cell for reproduction.