Which infectious agent is classified as a virus?

HIV, or Human Immunodeficiency Virus, is classified as a virus because it possesses the fundamental characteristics that define viral entities. Viruses are microscopic infectious agents that can only replicate inside the living cells of an organism. They are comprised of genetic material—either DNA or RNA—encapsulated in a protein coat and sometimes surrounded by a lipid envelope.

HIV specifically is an RNA virus that falls under the category of retroviruses. It uses reverse transcription to convert its RNA into DNA, which is then integrated into the host's genome. This ability to hijack the host's cellular machinery to reproduce makes HIV a prototypical example of a viral infectious agent.

In contrast, the other options represent bacterial or fungal pathogens. Escherichia coli and Streptococcus pyogenes are both types of bacteria, while Candida albicans is a yeast, a type of fungus. These organisms are distinct from viruses in that they are considered living cells capable of independent metabolism and growth outside of a host. Understanding these classifications is crucial for diagnosing infections and developing appropriate treatment strategies.