Final answer:
A virus is a type of malicious software that relies on other applications to execute and infect a system. Interferons are particularly effective against viral infections. The common cold is the most widespread virus in the human population.
Step-by-step explanation:
The malicious software that relies on other applications to execute and infect the system is known as a virus. Viruses attach themselves to legitimate programs or files and leverage the execution of these host applications to initiate their own execution. Unlike worms, which can self-replicate and spread independently, viruses require human action, such as running an infected program, to propagate.
In the context of the immune system and pathogens, interferons are particularly useful against infections with viruses. And regarding widespread viruses in the human population, the common cold virus (picornavirus) is much more prevalent than the human immunodeficiency virus (HIV) or Ebola.
In relation to acellular microorganisms, viruses consist of proteins and genetic material and are notably inert outside of a host organism. By incorporating themselves into a host cell's machinery, viruses can multiply and continue their lifecycle.