A virus may contain any of these four types of genetic material. The type of nucleic acid, and whether it is single- or double-stranded, determines how the virus integrates with the host organism. Different classes of viruses have different mechanisms for infection.

Note that retroviruses, which contain single-stranded RNA that is later converted to DNA, are often classified as a fifth type of virus.

There are a multitude of reasons as to why viruses are not classified as living organisms. In addition to the reasons given, viruses require host reproductive organelles and proteins in order to multiply. They also are not separated from the external world by a membrane or cell wall.

Human immunodeficiency virus (HIV), which is the causative agent of AIDS, primarily attacks the white blood cells. More specifically, it infects helper T-cells, macrophages, and dendritic cells. By damaging the cells of the immune system the virus creates an "immunodeficiency," weakening the body's ability to fight infection.

Virus particles, otherwise known as virions, always contain proteins. Whether they contain DNA or RNA depends solely upon the type of virion; different types of viruses will carry different types of nucleic acids. The capsids of virions contain proteins that surround the nucleic acids, whether it be DNA or RNA.

The question states that HIV is a RNA virus; therefore, the virus must possess an enzyme that transforms the RNA genome into DNA. This enzyme is called reverse transcriptase. Since it transforms RNA to DNA it is also called an RNA-dependent DNA polymerase.

DNA-dependent RNA polymerase (required for transcription) is not present in HIV because it is not a DNA virus.

Finally, HIV does contain DNA-dependent DNA polymerase. This enzyme is required for DNA replication, which is an important process in HIV replication.

Plus-strand RNA viruses can directly translate their RNA into proteins, however minus-strand RNA viruses require an additional step before translation. The minus-strand RNA is a complement to mRNA; therefore, minus-strand RNA must be transcribed into plus-strand RNA before translation. This step is the key difference between plus-strand and minus-strand RNA viruses.

You can eliminate the answer choices involving introns because most viruses do not contain introns. The viruses that contain introns could be either plus-strand or minus-strand RNA viruses (or DNA viruses).

The capsid of a virion is the region that contains the viral DNA or RNA and the necessary enzymes that facilitate infection of a host cell. When a virion infects a cell, the contents of the capsid are injected into the host cell. This marks the onset of infection. A capsid contains nucleic acids (DNA or RNA) and amino acids (enzymes).

A virion does not contain ribosomes or any other organelles. Instead, the virus must use host cell machinery to translate the proteins required for replication.

Viral genomes are incredibly diverse. Different types of viruses use different genetic materials. Viruses are capable of using double-stranded DNA, double-stranded RNA, single-stranded DNA, or single-stranded RNA.

Remember that reverse transcriptase is an enzyme that facilitates the generation of DNA from RNA. A virus containing reverse transcriptase must initially possess an RNA genome. Once inside the host cell the virus generates complementary DNA from the RNA template. You can eliminate the choices involving DNA.

A very long latent period of three years suggests that the virus undergoes the lysogenic cycle. The lytic cycle involves immediate lysis of the host cell and spread of infection after viral replication. In contrast, the lysogenic cycle causes the virus to become dormant. One of the main characteristics of a virus in the lysogenic cycle is that it has a long latent period.

Endoparasites, like the tapeworm, live in the body of their host and deprive the host of nutrients, often leading to sickness as seen in Daniel’s dog. Ectoparasites live and feed on the external surface of their host. Symbiotic relationships are those where two individuals are living in direct and intimate contact with one another, such as a parasite and its host. Mutualism and commensalism are a type of symbiotic relationship in which one or both species in the symbiotic relationship benefit and no one is harmed.