Chapter Checkpoints
These are some important ideas you are learning in Chapter 24:

Viruses in Infection and Disease
Viruses are particles of parasitic DNA or RNA. Most DNA viruses are double stranded, except for the single-stranded parvoviruses. RNA viruses are single stranded except for the double-stranded reoviruses. Viral infectivity requires a specific viral receptor site on the host cell, but most host tissues possess receptor sites for one or more viruses.

Viral diseases vary in severity, depending on viral virulence, age, health, and habitat of the human host. Lifelong immunity develops to some but not all viral agents. Virus infection can be diagnosed by overt symptoms, cultures, antigen detection, or with nucleic acid probes.

Most DNA and some RNA viruses can cause chronic infections and combine with the host genome. They also may have the potential to activate host oncogenes.

Survey of DNA Virus Groups
There are six major groups of DNA viruses. The enveloped DNA viruses include the poxviruses, herpesviruses, and hepadnaviruses. The non-enveloped group includes the adenoviruses, the papovaviruses, and the parvoviruses.

Enveloped DNA Viruses
Poxviruses are the largest and most complex of all viruses. They infect the skin, producing pustular lesions. Smallpox, which is now eradicated, molluscum contagiosum, monkeypox, and cowpox are the only poxviruses that infect humans.

Herpesviruses are large enveloped viruses that cause recurrent infections and are potentially carcinogenic in some cases. Herpes simplex 1 and 2 infect the skin and mucous membranes, but can cause encephalitis in neonates. The varicella-zoster is the causative agent of chickenpox and shingles. Cytomegalovirus (CMV) causes systemic disease in fetuses, neonates, immunosuppressed individuals and a type of mononucleosis. Epstein-Barr virus is the causative agent of Burkitt's lymphoma and infectious mononucleosis. Herpesvirus-6 causes roseola or ``forth disease.'' Other viruses are implicated in lymphomas sarcomas.

Hepadnaviruses are the causative agents of liver disease in many animals, including humans. Hepatitis B virus causes the most serious form of human hepatitis (hepatitis B or serum hepatitis). It is spread by direct contact, fomites, and mosquitoes. It is potentially lethal in a small percentage of cases, either by direct liver damage or by causing liver cancer. Other forms of viral hepatitis are caused by RNA viruses.

NonEnveloped DNA Viruses
Adenoviruses produce inflammatory infections in epithelial cells of the conjunctiva, intestine, respiratory tract, and lymph nodes. Papillomaviruses infect the skin and mucous membranes, producing warts. Genital warts are the leading STD in the US. The polyomaviruses transform infected host cells into tumors, which can be fatal in immunosuppressed hosts.

Parvoviruses: Nonenveloped Single-Stranded DNA Viruses
Parvoviruses are extremely small, resistant, single-stranded DNA viruses which cause diseases in cats, dogs, and humans. The most common virus, B19, is the cause of erythema contagiosum.

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