4. fill in the gaps in these sentences: 1)virology is often considered as a part 2)viruses can be classified according to infect. 3)another classification uses the geometrical shape of their the virus's structure. 4)a virus is a small can replicate only inside the living cells of organisms. 5)viruses are found in almost every 216 earth. 6)viruses cause a number of diseases in 7)viral infections in animals provoke 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 9)while viruses they don't engage in metabolism and depend on a host cell for reproduction.

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.