Chapter Checkpoints
These are some important ideas you are learning in Chapter 25:

Enveloped Segmented Single-Stranded RNA Viruses
This group includes the Orthomyxoviridae, the Bunyaviridae, and the Arenaviridae. Segmented viruses are able to reassort their genes during viral assembly, thereby continuously changing their antigenic structure, or antigenic drift. Thus, lasting immunity cannot be developed, because Orthomyxoviruses are continually changing their antigenic sites. This group includes the virus that causes influenza. When two genetic variants of the flu virus infect a host, they can combine to form a third genetically different strain. This event is termed antigenic shift. Like all enveloped RNA viruses, Orthomyxoviruses have two unique glycoprotein virulence factors in the envelope, hemagglutinin and neuraminidase. These assist in viral penetration and release from the host cell. Most orthomyxoviruses are spread by the aerosol route.

Bunyaviruses, are also referred to as Arboviruses because they are spread by insect bites. They cause potentially fatal encephalitis and hemorrhagic fevers. Arenaviruses are typically spread by rodents. They also cause fatal encephalitis and/or hemorrhagic fevers. Humans are considered accidental hosts, infected by direct contact or through aerosol transmission. Specific diseases caused by these viruses include Hantavirus hemorrhagic fever, Lassa fever, lymphocytic choriomeningitis, and California encephalitis.

Enveloped Non-segmented RNA Viruses
This group of RNA viruses includes the Paramyxoviridae and the Rhabdoviridae. Paramyxoviruses cause respiratory infections characterized by the formation of multinucleate giant cells. Paramyxovirus, Morbillivirus, and Pneumovirus are the principal disease agents in this group. They cause mumps, red measles, and RSV infections through aerosol transmission. The most serious of the Rhabdoviridae is the Lyssavirus, which causes rabies, a slow progressive, usually fatal, zoonotic disease of the CNS acquired by direct contact with infected mammals.

Other Enveloped RNA Viruses
This group of RNA viruses includes the Coronaviridae, agents of the common cold; Togaviridae, agents of rubella and arbovirus diseases, and Flavivirdiae, which cause yellow and dengue fevers. Arboviruses are viruses spread by insect vectors. They include the viral agents of encephalitis (St. Louis, Equine) and hemorrhagic fever.

Enveloped Single-Stranded RNA Viruses with Reverse Transcriptase: Retroviruses
Retroviruses are unique in their ability to create a double-stranded DNA copy from an RNA template using the enzyme reverse transcriptase. They are also called transforming viruses because the infected cells convert to cancer cells. Retroviruses infect many birds and mammals. Those infecting humans are named HTLV I, II, and HIV, and all attack various white blood cells, leading to cancer or immune dysfunction.

Acquired Immune Deficiency Syndrome (AIDS)
AIDS is caused by human immuno-deficiency virus (HIV). It is the most significant viral pandemic of this century, with 10?20 million estimated cases worldwide and 1 million known cases in the United States. It is acquired through direct contact with blood or sexual fluids in sufficient quantity. Those at greatest risk for infection include homosexual or bisexual males, IV drug users, heterosexual partners of AIDS carriers, recipients of blood products or tissue transplants, neonates form infected mothers, and medical/dental personnel.

HIV first infects macrophages, where it multiples and is shed, infecting all the macrophages, lymphocytes, and other cells that possess the CD4 receptor site. The most significant of these are the T4 (helper) cells which coordinate the immune responses to most infections. Also, the virus mutates rapidly, so that one individual is infected with hundreds of strains. Following infection, the virus often enters a dormant stage lasting from 2-15 years. AIDS is fatal because the host dies of opportunistic infections resulting from the destruction of T4 lymphocytes and stem cells. Infected macrophages also carry the virus to the CNS, causing neurological degeneration. Opportunistic infections involve fungi, protozoa, bacteria, and other viruses as well as Kaposi's sarcoma, an AIDS-associated cancer.

HIV infection is diagnosed by several tests. The most accurate is the Western blot test, which detects several different antibodies to HIV. There is no cure for AIDS, but improved supportive care has increased the life expectancy for many patients. It is difficult to develop drugs which attack the virus itself because of the high mutation rate. Protection against HIV includes following safer sex practices and taking increased precautions when handling needles and body fluids. Research continues for an effective AIDS vaccine, but many years of research and testing are necessary. There are also ethical considerations regarding trials of such vaccines in human populations.

Other Retroviral Diseases in Humans
Other retroviruses include the human T-cell leukemia viruses (HTLV I and II). They have been identified as the causative agents of ATL (adult T-cell leukemia) and HCL (hairy cell leukemia). Retroviruses have also been isolated from the blood of breast cancer, MS, and lupus patients.

Non-enveloped Single Stranded RNA Viruses: Picornaviruses and Caliciviruses
Picornaviruses cause a wide variety of viral infections including polio, aseptic meningitis, the common cold, hepatitis A, encephalomyocarditis, and foot-and-mouth disease. Major groups include the Enteroviruses, Rhinovirus, Cardiovirus, and Aphthovirus.

Non-enveloped Double-Stranded RNA Viruses: Reoviruses
Reoviruses are unusual in possessing both double-stranded RNA and two capsids. The two major groups are Rotaviruses and Reoviruses. Rotavirus is a significant intestinal pathogen for young children worldwide. Reovirus causes an upper respiratory infection similar to the common cold.

Slow Virus Infections by Unconventional Virus-like Agents
Long term degenerative and incurable CNS infections which cannot be linked to any known viral agent are thought to be caused by prions, infectious agents with atypical biological properties that are unusually resistant to destruction. Kuru and Creutzfeldt-Jakob disease are two of many TVDs (transmissible virus dementias) identified to date.

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