Epidemiology is the study of the patterns and spread of disease within a population. This term is related to epidemic, which is an outbreak of disease, that is, more than the usual number of cases in a given time period. An endemic disease is one that is present in a population, with an expected or usual number of cases in a given time. Influenza, for example, is endemic in large cities during the winter, and public health per-sonnel expect a certain number of cases. In some win-ters, however, the number of cases of influenza increases, often markedly, and this is an epidemic.
A pandemic is an epidemic that has spread throughout several countries. The bubonic plague pandemic of the 14th century affected nearly all of Europe and killed one-fourth of the population. Just after World War I, from 1918 to 1920, an especially virulent strain of the influenza virus spread around the world and caused at least 20 million deaths. More recently, an epidemic of cholera began in Peru in January 1991, but soon became a pandemic as cholera spread to neighboring South American countries.
To understand the epidemiology of a disease, we must know several things about the pathogen. These include where it lives in a host, the kinds of hosts it can infect, and whether it can survive outside of hosts.
The portal of entry is the way the pathogen enters a host (Fig. 22–1). Breaks in the skin, even very small ones, are potential portals of entry, as are the natural body openings. Pathogens may be inhaled, consumed with food and water, or acquired during sexual activ-ity. Most pathogens that enter the body by way of these natural routes are destroyed by the white blood cells found in and below the skin and mucous mem-branes, but some may be able to establish themselves and cause disease. You may wonder if the eye can be a portal of entry for other than eye infections, or you may already know that it is a portal for many respira-tory viruses. People may pick up cold or flu viruses on their hands and then rub their eyes; the viruses are then washed by tears into the nasal cavities, their pre-ferred site of infection.
QUESTION: Are there any portals of entry that are unlikely portals of exit?
Insects such as mosquitoes, fleas, and lice, and other arthropods, such as ticks, are vectors of disease. They spread pathogens when they bite to obtain a host’s blood. Mosquitoes, for example, are vectors of malaria, yellow fever, and encephalitis. Ticks are vectors of Lyme disease and Rocky Mountain spotted fever.
As mentioned previously, it is important to keep in mind that many hospital procedures may provide por-tals of entry for pathogens. Any invasive procedure, whether it involves the skin or the mucous membranes, may allow pathogens to enter the body. Thus it is essential that all healthcare workers follow aseptic technique for such procedures.
The portal of exit (see Fig. 22–1) is the way the pathogen leaves the body or is shed from the host. Skin lesions, such as those of chickenpox, con-tain pathogens that may be transmitted to others by cutaneous contact. Intestinal pathogens such as the hepatitis A virus and the cholera bacteria are excreted in the host’s feces, which may contaminate food or water and be ingested by another host (this is called the fecal–oral route of transmission). Respiratory pathogens such as influenza and measles viruses are shed in respiratory droplets from the mouth and nose and may be inhaled by another person. The pathogens of the reproductive tract, such as the bacte-ria that cause syphilis and gonorrhea, are transmitted to others by sexual contact. Notice that with respect to epidemiology, the pathogen travels from one host’s portal of exit to another host’s portal of entry.
Some pathogens cause disease only in people. Measles, whooping cough, syphilis, and bacterial meningitis are strictly human diseases. To acquire such a disease, a person must be exposed to someone who has the illness.
Also of importance is that upon recovery from some diseases, the host may continue to harbor the pathogen and thus be a reservoir of it for others. Such a person is called a carrier. Diseases for which the car-rier state is possible include typhoid, diphtheria, and hepatitis B.
Many other diseases, however, are really animal diseases that people acquire in certain circumstances. These diseases are calledzoonoses (singular: zoono-sis) and include plague, Lyme disease, encephalitis, and Rocky Mountain spotted fever, which are spread from animal to person by vectors such as ticks or fleas. Rabies is acquired by contact with infected animal saliva or infected tissue, the virus entering the new host through breaks in the skin. Salmonellosis is a type of food poisoning caused by the intestinal bacilli of animals that contaminate meats such as chicken and turkey. Prevention of such diseases depends upon knowledge of how they are spread. For example, peo-ple who live in areas where Lyme disease is endemic should be aware that the disease is acquired by way of a tick bite. If children and pets are examined for ticks after they have been out of doors, the chance of acquiring Lyme disease is greatly diminished.
Some bacteria are pathogenic only by accident, for their natural habitat is soil or water, where they act as decomposers. The bacteria that cause gas gangrene, tetanus, and botulism are normal soil flora and cause disease when they (or their toxins) contaminate a skin wound, or, in the case of botulism, the toxin is present in food.
Based on our knowledge thus far, we can classify infec-tious diseases as noncommunicable or communicable. A noncommunicabledisease is one in which a resi-dent species causes disease under certain conditions or in which a non-resident species causes disease when it enters the body. Such diseases cannot be transmitted directly or indirectly from host to host. Cystitis caused by E. coli in a hospital patient, for example, is not com-municable to the nurses who care for that patient. Similarly, a nurse caring for a patient with tetanus or botulism need not worry about acquiring these dis-eases; both are noncommunicable.
A communicable disease is one in which the pathogen may be transmitted directly or indirectly from host to host. Direct spread of infection is by way of cutaneous contact (including sexual contact), respi-ratory droplets, contaminated blood, or placental transmission from mother to fetus. Indirect spread is by way of contaminated food or water, or vectors, or fomites, which are inanimate objects that carry the pathogen. Influenza and cold viruses, for example, can survive outside their hosts for a time, so that objects such as eating utensils, door knobs, or chil-dren’s toys may be vehicles of transmission for these pathogens.
Some communicable diseases may also be called contagious, which means that they are easily spread from person to person by casual cutaneous contact or by respiratory droplets. Chickenpox, measles, and influenza are contagious diseases. In contrast, AIDS is not contagious, because sexual contact, blood contact, or placental transmission is necessary to acquire the virus (HIV). HIV is not spread by cutaneous contact or by respiratory droplets.
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