The number of viral infections worldwide is nearly impossible to measure accurately. Certainly, viruses are the most common cause of acute infections that do not result in hospitalization, especially when one considers widespread diseases such as colds, measles, chicken pox, influenza, herpes, and warts. If one also takes into account prominent viral infections found only in certain regions of the world (dengue fever, Rift Valley fever, and yellow fever), the total could easily exceed several billion cases each year. Although most viral infections do not result in death, some, such as rabies, HIV, and Ebola, have very high mortality rates, and others (polio, hepatitis) can lead to long-term debility. Continuing research is focused on the connection of viruses to chronic afflictions of unknown cause, such as type 1 diabetes, multiple sclerosis, and various cancers.
Because some viral diseases can be life threatening, it is essential to have a correct diagnosis as soon as possible. Obtaining a complete clinical picture of the disease (specific signs) is often the first step in diagnosis. This may be followed by identification of the virus in clinical specimens by means of rapid tests that detect the virus or signs of cytopathic changes in cells or tissues (see figure 1). Immunofluorescence techniques or direct examination with an electron microscope are often used for this. Samples can also be screened for the presence of indicator molecules (antigens) from the virus itself. A standard procedure for the detection of many viruses is the polymerase chain reaction (PCR), which can detect and amplify minute amounts of viral nucleic acid in a sample. In certain infections, definitive diagnosis requires cultivation of the virus using cell culture, embryos, or animals, but this method can be time-consuming and slow to give results. Screening tests can detect specific antibodies that indicate signs of virus infection in a patient’s blood. This is the primary test for the detection of HIV.
Fig1. Cytopathic changes in cells and cell cultures infected by viruses. (a) A multinucleate giant cell (center) infected with the measles virus (inset). (b) Multiple small, dark inclusions are clearly visible within the nuclei of cultured cells infected with the varicella-zoster virus (500×). (a and a inset): CDC; (b): Source: CDC
The nature of viruses has at times been a major impediment to effective therapy. Because viruses are not bacteria, antibiotics aimed at bacterial infections do not work for viruses. Although more antiviral drugs are being developed, most of them block virus replication by targeting the function of host cells. This can cause severe side effects. Antiviral drugs are designed to target one of the steps in the viral life cycle you learned about earlier in this chapter. Drugs used to treat HIV, for example, include enfuvirtide, which blocks the fusion of HIV with a host cell; epivir, which blocks viral DNA synthesis; and protease inhibitors, which interfere with final assembly and re lease of the virus. Another compound that shows some potential for treating and preventing viral infections is a naturally occurring hu man cell product called interferon. Vaccines that stimulate immunity are an extremely valuable tool but are available for only a limited number of viral diseases.
