Emerging viruses arise by

(A) mutation of existing viruses.

(B) the spread of existing viruses to new host species.

(C) the spread of existing viruses more widely within their host

species.

(D) all of the above.

A mutation is a process that involves alteration in the sequence of genetic codes of an organism by changing its genomic sequence. Viruses mutate very rapidly due to the absence of proofreading ability in the RNA polymerase. New strains of viruses emerge due to the mutation in pre-existing viruses.

When an existing virus undergoes a mutation, it emerges as a more deadly and harmful virus. Alteration in genetic structure causes the production of new emergent viruses.

Thus, themutation in the existing virus can arise new emergent virus. However, new strains also arise by spreading viruses to new hosts.

Therefore, the given option is false.

A new disease can emerge when it is disseminated from an isolated small population. When this disease is in a small population, it cannot be noticed, but when it is disseminated in a new host, it can become emergent.

Thus,the spread of the virus to a new host from the existing virus is the cause of the emerging virus.

Therefore, the given option is false.

A new strain of the virus also emerges when an existing virus spreads more among a host population. When a host is infected with more than one virus strain, new strains are also produced through genetic recombination.

Thus,new viruses can emerge when they are present more widely in their host species.

Therefore, the given option is false.

All the above reasons for the rise of an emergent virus are correct because these are the main reasons that create new harmful strains of the viruses.

Thus, all the above reasons for emerging viruses are correct.

Therefore, the given option is true.