What is a tiny, nonliving particle that invades and reproduces inside a living cell?

A virus is indeed a tiny, nonliving particle that invades and reproduces inside living cells. Viruses are composed of genetic material—either DNA or RNA—surrounded by a protein coat. Unlike bacteria, which are single-celled organisms and have a cellular structure, viruses lack the components necessary for metabolic processes and cellular organization. This makes them nonliving outside of a host organism.

When a virus enters a living cell, it hijacks the cell's machinery to replicate itself and produce more viral particles. This process often damages or destroys the host cell in the process. The ability of viruses to reproduce only within living cells distinguishes them from other pathogens such as bacteria, prions, and fungi. Bacteria are living organisms that can reproduce independently, prions are misfolded proteins that induce abnormal folding of normal proteins, and fungi are multicellular or unicellular organisms that have their own cellular structure.

Understanding the nature of viruses is key in virology and medicine, particularly in the development of vaccines and antiviral therapies.