Final answer:
A preexisting virus can become an emerging virus through antigenic drift, which involves small genetic mutations, or antigenic shift, where there is a significant genetic reassortment between different virus strains. Environmental factors and modern transportation also contribute to the spread of new viruses.
Step-by-step explanation:
A preexisting virus can become an emerging virus in several ways.
The first way involves a process called antigenic drift, where random mutations in the virus's genetic material cause small changes in its surface proteins. These changes may allow the virus to evade the host's immune system or infect a different species. These mutations are common during the virus replication cycle and can lead to the virus binding to cell-surface receptors in new hosts, particularly in environments like poultry and swine farms where animals and humans are in close contact.
Another way is through antigenic shift, which is a significant genetic reassortment that occurs when a virus exchanges genetic material with another strain. This can create a new virus subtype with major changes in its surface proteins, to which the general population has little or no immunity. Such shifts can lead to highly virulent and transmissible viruses, capable of causing pandemics.
Finally, factors like modern transportation or environmental changes can also drive the emergence of new viruses by increasing contact between different species and enabling the rapid spread across large geographic areas, as seen with diseases like West Nile Virus, dengue fever, and malaria.