A surprising number of new infectious agents continue to be discovered, and there are several reasons for this: • Some pathogens were discovered due to improved methods of detection, although they have likely been present in humans for centuries. For example, Helicobacter pylori, which causes gastritis and peptic ulcer disease, was only discovered in the 1980s. More recently, a new cause of leprosy was discovered, Mycobacterium lepromatosis. This agent, identified by sequencing bacterial DNA from biopsy material of patients who died of leprosy, is a close relative of the previously known
Mycobacterium leprae.
Animals are a source of new pathogens that infect humans. Two coronaviruses that cause severe respiratory tract infections in humans, Middle East respiratory syndrome coronavirus (MERS CoV) and the severe acute respiratory syndrome (SARS) virus, likely spread to humans from animals and were first detected in 2003 and 2012, respectively. Other examples of pathogens that emerged in humans after being transmitted from animals include HIV and B. burgdorferi.
Microorganisms can acquire genes that enhance virulence or overcome host defense. In 2011, almost 4000 individuals in Germany were infected with a new strain of highly virulent shiga-toxin producing Escherichia coli that spread in sprout seeds. The new strain was derived from a different type of E. coli that acquired a gene for the shiga-toxin from a bacteriophage.
Other pathogens have become much more common because of immunosuppression caused by AIDS, or therapy to prevent transplant rejection or treat cancers (e.g., human herpes virus 8, Mycobacterium avium complex, P. jirovecii).
Additional clinical syndromes may be recognized after the pathogen has been known for some time, possibly due to new contributing factors. Although Zika virus was discovered in 1947 in Uganda, very few human cases were reported for many years, until the virus more recently spread through additional countries, expanding the range from Africa, Asia, and the Pacific to the Americas, causing newly recognized clinical cases. The virus is contracted primarily via the Aedes species mosquito but also via sexual transmission and possibly blood transfusion. Although many individuals infected have no or mild nonspecific symptoms, infection may be associated with Guillain-Barre syndrome, a form of rapid-onset muscle weakness caused by immune system damage to the nervous system. Infection with Zika virus during pregnancy is associated with birth defects, including microcephaly. Many more congenital cases were diagnosed in the 2015 outbreak beginning in Brazil compared to earlier outbreaks, creating a Public Health Emergency of International Concern. Further investigation is ongoing to determine if other variables may be contributing to the CNS effects such as genetics, co-infections, immunity from past infections, or yet to be defined environmental factors.
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Several factors contribute to the emergence of infectious diseases:
Human behavior affects the spread and demographics of infections. AIDS was first recognized in the United States as predominantly a disease of homosexual men and drug abusers, but heterosexual transmission is now more common. In sub-Saharan Africa, the area of the world with the highest number of AIDS cases, it is predominantly a heterosexual disease. The Ebola virus epidemic of 2014 spread to more countries than previous Ebola outbreaks due in part to the frequent movement of people across borders in West Africa, as well as burial practices that involved contact with the bodies of the deceased. SARS, mentioned earlier, spread very quickly to 24 countries due to human air travel before it was contained.