An emerging infectious disease (EID) is an infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human pathogens. EIDs are caused by newly identified species or strains (e.g. Severe acute respiratory syndrome, HIV/AIDS) that may have evolved from a known infection (e.g. influenza) or spread to a new population (e.g. West Nile fever) or to an area undergoing ecologic transformation (e.g. Lyme disease), or be "reemerging" infections, like drug resistant tuberculosis. Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and non-infectious conditions leading to the development of novel syndemics. Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.
The U.S. Centers for Disease Control and Prevention (CDC) publishes a journal "Emerging Infectious Diseases" that identifies the following factors contributing to disease emergence:
- Microbial adaption; e.g. genetic drift and genetic shift in Influenza A
- Changing human susceptibility; e.g. mass immunocompromisation with HIV/AIDS
- Climate and weather; e.g. diseases with zoonotic vectors such as West Nile Disease (transmitted by mosquitoes) are moving further from the tropics as the climate warms
- Change in human demographics and trade; e.g. rapid travel enabled SARS to rapidly propagate around the globe
- Economic development; e.g. use of antibiotics to increase meat yield of farmed cows leads to antibiotic resistance
- Breakdown of public health; e.g. the current situation in Zimbabwe
- Poverty and social inequality; e.g. tuberculosis is primarily a problem in low-income areas
- War and famine
- Bioterrorism; e.g. 2001 Anthrax attacks
- Dam and irrigation system construction; e.g. malaria and other mosquito borne diseases
The U.S. National Institute of Allergy and Infectious Diseases recognizing the following emerging and re-emerging diseases as of 2004.
Newly recognized (since the 1980s):
- Australian bat lyssavirus
- Babesia, atypical
- Bartonella henselae
- Coronaviruses, including SARS coronavirus
- Encephalitozoon cuniculi
- Encephalitozoon hellem
- Enterocytozoon bieneusi
- Helicobacter pylori
- Hendra virus (equine morbilli virus)
- Hepatitis C
- Hepatitis E
- Human herpesvirus 8
- Human herpesvirus 6
- Lyme borreliosis
- Parvovirus B19
- Coccidioides immitis
- Enterovirus 71
- Prion diseases
- Streptococcus, group A
- Staphylococcus aureus
Diseases with bioterrorism potential, CDC category A (most dangerous):
- Clostridium botulinum
- Smallpox and other pox viruses
- Viral hemorrhagic fevers
- Arenaviruses: Lymphocytic choriomeningitis (LCM), Junin virus, Machupo virus, Guanarito virus, Lassa Fever
- Bunyaviruses: Hantaviruses, Rift Valley Fever
- Flaviviruses: Dengue
- Filoviruses: Ebola, Marburg
- Yersinia pestis
Diseases with bioterrorism potential, CDC category B:
- "Brucella" species (brucellosis)
- "Burkholderia pseudomallei" (melioidosis)
- "Burkholderia mallei" (glanders)
- "Coxiella burnetii" (Q fever)
- Epsilon toxin of "Clostridium perfringens"
- Food-borne and Water-borne Pathogens
- "Campylobacter jejuni"
- Diarrheagenic "E. coli"
- "Listeria monocytogenes"
- Pathogenic vibrios
- "Shigella" species
- "Yersinia enterocolitica"
- "Cryptosporidium parvum"
- "Cyclospora cayetanensis"
- "Entamoeba histolytica"
- "Giardia lamblia"
- Hepatitis A
- Mosquito-borne encephalitis viruses
- California encephalitis
- Eastern equine encephalitis (EEE)
- Japanese encephalitis virus (JE)
- Kyasanur Forest virus
- LaCrosse virus (LACV)
- Venezuelan equine encephalitis (VEE)
- Western equine encephalitis (WEE)
- West Nile virus (WNV)
- Ricin toxin (from "Ricinus communis")
- Staphylococcal enterotoxin B
- Typhus fever ("Rickettsia prowazekii")
Diseases with bioterrorism potential, CDC category C (least dangerous):
- Multidrug-resistant tuberculosis (MDR-TB)
- Nipah virus
- SARS coronavirus
- Tick-borne encephalitis virus
- Tick-borne hemorrhagic fever viruses
- Crimean-Congo hemorrhagic fever virus
- Yellow fever
- Other hantaviruses
- Other rickettsias
Since 2004, NIAID has added to its biodefense emerging pathogen list:
- "Yersinia pestis" (plague, category A)
- Chapare virus (category A areanavirus)
- Lujo (category A arenavirus)
- Chlamydia psittaci (category B)
- "Naegleria fowleri" (category B)
- "Balamuthia mandrillaris" (category B)
- St. Louis encephalitis virus (SLEV, category B)
- Tick-borne hemorrhagic fever viruses (category C)
- Bunyaviruses: Severe Fever with Thrombocytopenia Syndrome virus (SFTSV), Heartland virus
- Flaviviruses: Omsk Hemorrhagic Fever virus, Alkhurma virus, Kyasanur Forest virus (reclassified from B to C)
- Powassan virus (Deer Tick virus, category C)
- Chikungunya virus (category C)
- "Coccidioides" species (category C)
- Human coronavirus HKU1 (category C)
- Middle East respiratory syndrome coronavirus (category C)
- BK virus
- Bordetella pertussis
- Borrelia miyamotoi
- Clostridium difficile
- Cryptococcus gattii
- Enterococcus faecium
- Enterococcus faecalis
- Enterovirus 68
- JC virus
- Mumps virus
- Rubeola (measles)
- Zika virus
NIAID also monitors antibiotic resistance, which can become an emerging threat for many pathogens.
Methicillin-resistant "Staphylococcus aureus"
Methicillin-resistant Staphylococcus aureus (MRSA) evolved from Methicillin-susceptible Staphylococcus aureus (MSSA) otherwise known as common "S. aureus". Many people are natural carriers of "S. aureus", without being affected in any way. MSSA was treatable with the antibiotic methicillin until it acquired the gene for antibiotic resistance. Though genetic mapping of various strains of MRSA, scientists have found that MSSA acquired the mecA gene in the 1960s, which accounts for its pathogenicity, before this it had a predominantly commensal relationship with humans. It is theorized that when this "S. aureus" strain that had acquired the mecA gene was introduced into hospitals, it came into contact with other hospital bacteria that had already been exposed to high levels of antibiotics. When exposed to such high levels of antibiotics, the hospital bacteria suddenly found themselves in an environment that had a high level of selection for antibiotic resistance, and thus resistance to multiple antibiotics formed within these hospital populations. When "S. aureus" came into contact with these populations, the multiple genes that code for antibiotic resistance to different drugs were then acquired by MRSA, making it nearly impossible to control. It is thought that MSSA acquired the resistance gene through the horizontal gene transfer, a method in which genetic information can be passed within a generation, and spread rapidly through its own population as was illustrated in multiple studies. Horizontal gene transfer speeds the process of genetic transfer since there is no need to wait an entire generation time for gene to be passed on. Since most antibiotics do not work on MRSA, physicians have to turn to alternative methods based in Darwinian medicine. However prevention is the most preferred method of avoiding antibiotic resistance. By reducing unnecessary antibiotic use in human and animal populations, antibiotics resistance can be slowed.