Are viruses alive? This question is debated among scientists throughout the world. Consider the following passage. Scientific researchers discovered agents that behaved like bacteria causing diseases such as rabies and hoof-and-mouth, but were much smaller. At this time it became the general view that viruses were biologically "alive." This perception changed in 1935 when the tobacco mosaic virus was crystallized and scientists demonstrated that the particles lacked any mechanisms necessary for metabolic function. It was determined that viruses consisted of a nucleic acid, DNA or RNA, surrounded by a protein shell and the scientific view changed: viruses are complex biochemical mechanisms but not alive.
According to the passage, the MOST IMPORTANT evidence supporting the opinion that viruses are not alive is that they
A) cause disease.
B) contain DNA or RNA.
C) are much smaller than bacteria.
D) do not have a cellular structure.

Viruses do not possess their own metabolism, so they require a host machinery to replicate. So they can not independently replicate outside of a host cell. They integrate their genetic material into host DNA in order to replicate their DNA.

They are not considered to be alive as they do not require energy and metabolism as they do not grow and produce waste products and they do not respond to external stimulus. Since they are not considered to be living.

Example : Human Immuno Virus