A bacteriophage is a virus which attacks bacteria and consists of a head, tail, and tail fibers all made of protein. The head contains genetic material (usually linear DNA) which is injected into the host bacterium upon infection. The protein parts do not enter the bacterium, and it does not have any ribosomes or organelles of its own.

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.

Reverse transcriptase binds RNA and catalyzes the formation of a complementary DNA molecule. Since it needs an RNA template to work, this enzyme is RNA dependent. Since it forms a DNA molecule, this enzyme is a DNA polymerase. Recall the chemical difference between DNA and RNA - DNA lacks a hydroxide group at the 3' position of the ribose sugar, which makes it deoxyribose.

Eukaryotic cells have a nuclei; prokaryotic cells have no nuclei.

Eukaryotic cells are considerably larger and more complex. Eukaryotic cells possess membrane-bound organelles; prokaryotic cells, which are smaller and less complex, do not. Both eukaryotic and prokaryotic cells can have flagella, but not necessarily in all cases.

Note that ribosomes are not membrane-bound, and are found in both prokaryotes and eukaryotes.

The only disease out of these that is caused by a prion is mad cow disease, and is therefore the correct answer. HIV is caused by a virus, syphilis and tuberculosis by bacteria, and lupus is an autoimmune disorder.

The lysogenic cycle occurs when a virus incorporates its DNA into the host genome. It stays inside the host genome, getting replicated along with the host until conditions are right when it excises itself and beings the process of hijacking the cell to make more virus. Both retroviruses and bacteriophages can undergo this process.

Some viruses have a DNA genome, while others use RNA. While DNA may be a crucial component to determining life, it cannot be used to differentiate viruses from living organisms because some viruses do carry their own DNA.

All viruses can be differentiated from living organisms because they do not "eat" or absorb nutrients, they do not have cells, they do not respond to stimuli, and they do not reproduce independently. All living organisms absorb nutrients, have cells, respond to stimuli, and are capable of reproduction, thus making viruses different.

The three main media of attenuating a virus are infecting birds, horses, or cultured cell lines. West Nile Virus is an interesting virus in that host and susceptible organisms happen to be birds, horses, and humans. This virus also does not seem to replicate in other animal reservoirs such as lizards. It does not replicate in mosquitos. Moreover, recent attempts to culture a neuronal cell line to replicate the virus have been unsuccessful due to the extended length of life cycle of neuronal cells.

Reverse transcriptase is a viral enzyme used by some viruses to transcribe double-stranded DNA from their single-stranded RNA, which is the exact opposite of the normal transcription process of the cell. The double-stranded viral DNA is then able to integrate into the double-stranded DNA of the infected cell’s genome so that whenever the cell divides, all of its daughter cells will carry the viral DNA. This viral DNA can eventually be transcribed by the cell’s own enzymes to produce viral RNA. Some of the viral RNA will be translated into viral proteins, and new viruses will be assembled from these products.

Viruses are unique organisms in that they have a relatively simple cell structure. They have an outer protein coat, which sets them apart from other organisms. Peptidoglycan is found in bacteria, lipid bilayer is found in eukaryotic cells, and chitin is found in fungi. Membrane-bound orangelles are found in more complex cells, such as eukaryotic cells, and are not unique to viruses.