13.17: Viruses are responsible for many health problems.

Many diseases are caused by viruses. Some viral diseases have been responsible for worldwide epidemics, called pandemics. The influenza pandemic of 1918–1919 killed at least 20 million people, and possibly as many as 50 million. In the current HIV/AIDS pandemic, more than 75 million people have been infected and about 35 million people have died of AIDS. Worldwide, about 1.6 million people died of AIDS-related illnesses in 2012.

Other viral diseases, such as the common cold, are not usually serious. Herpes is another common viral infection in humans, caused by two related viruses. Oral herpes is an infection near the mouth and can cause cold sores lasting two to three weeks. More than three-quarters of all adults in the United States are infected with the virus, with more than 50 million people experiencing outbreaks each year. Genital herpes, a sexually transmitted disease, affects about one in six people between the ages of 14 and 49 in the United States. The viruses causing herpes are present in and released from sores and spread from one person to another by skin-to-skin contact, but an infected person can also pass on the virus even when she or he has no symptoms. Although a variety of treatments are available for both types of herpes, including antiviral drugs that can reduce the severity and duration of symptoms, there is currently no cure.

DNA viruses have base-pair sequences that are stable over time, because the enzymes that replicate the DNA check for errors and correct them during replication. From a health perspective, the fact that DNA viruses do not change rapidly makes it much easier to fight and treat diseases caused by these viruses, using vaccines. Vaccination against a DNA virus such as smallpox, for example, provides years of protection.

RNA viruses, by contrast, change quickly, because the enzymes that carry out RNA replication do not have error-checking mechanisms. As a consequence, because the RNA-replicating enzymes make errors as they assemble new RNA molecules, RNA viruses are continuously mutating into new forms. The common influenza virus that causes outbreaks of flu every year, for example, is an RNA virus. And because flu viruses mutate so fast, the virus that causes health problems changes from one flu season to the next, necessitating a different flu vaccine every year.

About every 50 years, a new variety of influenza causes a pandemic. The 20th century had three major influenza pandemics, and all of them originated in the same peculiar way. In each of these pandemics, a bird flu virus gained the ability to infect human cells by passing through pigs, and then the virus spread rapidly through the human population worldwide. The most famous of these pandemics was that of 1918–1919, mentioned above (FIGURE 13-26). It was called the Spanish influenza, because it seemed to enter Europe through Spain, but it originated in Asia. The Asian flu and Hong Kong flu pandemics also originated in Asia, as did smaller viral outbreaks, such as Korean flu (1947), swine fever (1976), and SARS (severe acute respiratory syndrome, 2002–2003), which did not become pandemics.