Clinical Virology

2.1. DNA and RNA Viruses and Human Disease

Viruses are infectious particles generally composed only of nucleic acid surrounded by a protein coat (see Fig. 1). Viruses survive by infecting a host cell, where viral proteins take over host cellular machinery to produce viral progeny. The host cell may be killed in the process, as in the case of most adenoviral infections, or harbor a latent viral infection, such as herpes simplex virus (HSV), for many years. The type of nucleic acid and the presence or absence of an envelope categorizes viruses (see Table 1). The nucleic acids may be DNA or RNA and double-stranded, single-stranded, sense, or antisense.

Analysis of viral loads, which estimates the virus particles present in the host serum at a given time, may be very important in diagnosing, treating, and monitoring viral illness. In particular, viral loads may be very useful in detecting the presence of latent infections, when low copy numbers of virus are present (as in early viral illness) and to determine response to antiviral therapy.

2.2. Clinical Utility of Viral Loads

Human cytomegalovirus (CMV) is a DNA virus associated with significant human disease. CMV is a pathogen in immunocompromised (1) indi-

Table 1

Classification of Viruses Associated with Human Disease

Table 1

Classification of Viruses Associated with Human Disease

Virus family

Nucleic acid



Example of human disease


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