A disease is considered epidemic when the number of cases rises above what would normally be expected in a given area. Epidemics have posed a perpetual threat to human health throughout history. Despite remarkable advances in infectious disease prevention and treatment, epidemics continue to threaten populations as new diseases emerge and some old ones return. The ease of travel also contributes to communicable disease spread in an increasingly global world. While most epidemics involve an infectious cause, changing behavior patterns have led to epidemic levels of some chronic diseases.
Infectious diseases such as polio, cholera, smallpox and tuberculosis have historically caused sporadic epidemics of devastating proportions. These epidemic diseases are frequently spread through person-to-person contact, but they also commonly occur due to contaminated food. Despite food safety precautions, sporadic outbreaks of norovirus, salmonellosis, shigellosis, listeriosis and E.coli remain common in the United States. Contaminated eggs, meat, fish, poultry, milk products, and fresh fruits and vegetables are often the culprits for these food-borne epidemics. Sporadic disease outbreaks can also be caused by exposure to pests, as with mosquitoes causing yellow fever and malaria epidemics. Toxin exposure is another possible culprit, such as the spike in leukemia among atomic bomb survivors due to radiation exposure.
Some communicable disease epidemics are cyclic, with spikes in cases occurring seasonally or in another predictable pattern. Influenza, or the flu, is a common example. In the United States, flu season occurs during the winter months, typically reaching epidemic levels in a matter of weeks. Global flu epidemics -- known as pandemics -- are particularly devastating, such as the so-called swine flu pandemic of 2009-2010. Other infectious diseases that may cause cyclic epidemics include meningococcal meningitis, measles and rotavirus, a diarrheal illness. Vaccines against these diseases substantially reduce the possibility of epidemics when sufficient numbers of people are fully vaccinated.
Emerging Infectious Diseases
While many infectious diseases can be prevented or controlled with immunizations and antibiotics, several newly recognized diseases have emerged since the mid-20th century. These diseases include HIV/AIDS, Ebola, hantavirus, Middle East respiratory syndrome, and severe acute respiratory syndrome, or SARS, which have posed unfamiliar and dangerous threats. For example, SARS emerged in Asia in February 2003 and quickly spread to 2 dozen countries, sickening nearly 8,100 people and causing almost 800 deaths, according to the World Health Organization. Other diseases -- such as malaria, tuberculosis and certain types of bacterial pneumonia -- are reemerging epidemic threats as the causative microorganisms have gained resistance to once reliable treatments.
Although the traditional definition of epidemics refers to infectious diseases, the rates of many noncontagious diseases have risen to epidemic proportions. These diseases are often tied to changing behavior patterns that are detrimental to personal health. For example, the rate of obesity among adults in the United States more than tripled from 1960 to 2010, according to the Centers for Disease Control and Prevention. This trend toward increasing overweight and obesity -- both in the United States and around the word -- has been tied to epidemic levels of type 2 diabetes along with increasing numbers of other related diseases.