Answer the following question given this paragraph: 

A virus is a sub-microscopic particle that can infect living cells. Viruses are much smaller than prokaryotic organisms. In essence, a virus is simply a nucleic acid surrounded by a protein coat, as seen in the Figure below . This outer coat is called a capsid . Viruses will be discussed in more detail in the Viruses concepts.


Q: In essence, a virus is simply a nucleic acid surrounded by what?


A:
a protein coat