Dangerous Microbes

  • Bjørg Marit Andersen


The most dangerous microbes for humans are those that are easily transmitted, virulent and invasive to central organs like the blood and lung, robust survivors in the environment, have a low infection dose and are without any specific treatment or vaccine. Most of them are zoonoses transmitted from animals and often with insects as vectors. The most dangerous microbes cause a very high mortality, are identified as high-risk agents or “biohazard-level 4” agents and are treated at the highest level of infection protection with strict isolation measures. Dangerous microbes occur as a problem mostly in countries with low hygiene standards/high population density and in tropical-subtropical areas. Infection control must always be based on hygienic measures and strict infection protection.

This chapter is a short information about the most virulent and pathogenic agents, geographic area and severity of disease.


Microbes Dangerous microbes Geographic area of virulent agents Dangerous virus and bacteria Zoonosis VHF Anthrax Plague Biosafety level 4 Infection control Survivors High-risk microbes Severity of disease 

80.1 Dangerous Microbes

New, emerging or re-emerging infectious diseases and agents still appear. More than 15 new potential human pathogenic microbe types may be registered every year. New infectious diseases and emerging virulence properties still appear, for example, avian influenza viruses (H5N1 and others), HTLV and other retroviruses, HPV, Sindbis virus, Parvovirus, Bocavirus, Coronavirus, or bacteria like Legionella, Borrelia, group A streptococci and meningococci [1, 2, 3, 4, 5].

Virus not vaccine preventable or without specific treatment option, low infection dose and high capacity to survive in the environment will be a major problem if associated with incurable disease, disability or death. Virulent and dangerous microbes occur as a problem mostly in countries with low hygiene standards/high population density and in tropical-subtropical areas. Infection control must be based on hygienic measures and proper infection protection [2, 3, 4, 5, 6, 7].

The most dangerous microbes for humans are those that are easily transmitted, virulent and invasive to the central organs like the blood and lung, robust survivors in the environment, have a low infection dose and are without any specific treatment or vaccine [2, 3, 4, 5]. Most of them are zoonosis transmitted from animals and often with insects as vectors [8, 9, 10, 11, 12]. Dangerous microbes have also been used as biological weapons like anthrax, plague, botulism, Coxiella, Brucella, viral haemorrhagic fever (VHF) viruses, poxviruses and a number of other unusual agents [4, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]. In the United States, there was an anthrax bioterrorism attack in 2001 [18, 19].

If the infectious agent and transmission ways are unknown and the situation is uncontrollable, it is important to follow pre-planned practical measures like strict isolation in negative air pressure isolation, registering of all contacts and the use of personal protective equipment (PPE) for air- and contact-borne infection [4, 8, 9, 10, 11, 12, 13, 17, 20, 24, 26, 27, 28, 29].

80.1.1 Dangerous Virus

During the last 20 years, there have been several large outbreaks of new and re-emerging viruses. Examples are systemic acute respiratory syndrome (SARS) (2002–2003), avian influenza viruses (H5N1, 2005), pandemic influenza (2009), Middle East respiratory syndrome (MERS) (2012) and Ebola (2014 and 2016) [1, 4, 5, 8, 11, 12].

VHF virus and a number of other special viruses are considered to cause the world’s most dangerous infections with very high mortality, lack of therapeutic possibilities and often absence of effective vaccines. Such viruses are identified as high-risk agents or “biohazard-level 4” agents and are treated at the highest level of infection protection with strict isolation measures [4, 11, 12, 26, 27, 28].
  • Virus families, with members especially dangerous to humans, are Arenaviridae, Filoviridae, Flaviviridae, Bunyaviridae, Togaviridae and Paramyxoviridae (Nipah encephalitis virus, etc.) and several others. They are mostly vector borne by insects and animals (zoonosis), dependent on a tropical-subtropical climate and temperature. Therefore, the colder zones in Europe, America, Asia and Australia are nearly free from more such serious infectious agents (see the table below) [1, 2, 4, 11, 12].

  • Some very virulent viruses may be spread on all continents: TBE, RSSE, CCHF, rabies virus (Rhabdoviridae), avian influenza virus (AH5N1) and other high-pathogenic influenza viruses (Orthomyxoviridae). SARS and MERS (Coronaviridae) may be spread globally as nosocomial infections. MERS is a newly discovered coronavirus-zoonosis (dromedaries, bats, etc.) and acts as SARS, also with a tendency for nosocomial spread in hospitals, still going on in the Middle East. [30] These and other viruses are adapted to the climate and spread by direct transmission person-to-person, by increased mobility in the population and/or spread via new vectors, like animals, birds and insects [1, 4].

  • A few of these dangerous viruses are vaccine preventable, and specific antiviral therapy is lacking in most cases. Other New Virulent Viruses

  • Zika virus—newly discovered flavivirus in South America, with mild to serious symptoms and teratogenic effect [11].

  • Polio-like illnesses, especially among children, discovered in August 2014 in the United States. Probably caused by enterovirus D68 [31].

  • HIV aggressive variants (CRF19) have been detected in Cuba in 2015 and earlier in Africa, with a faster course from infection to AIDS development [32].

80.1.2 Dangerous Bacteria

Bacteria resistant to antibacterial drugs have developed rapidly among staphylococci (MRSA), enterococci (VRE), tuberculosis bacilli (multidrug-resistant mycobacteria) and gram-negative rods (ESBL) in the last 30–50 years. From 2009 to 2010 onwards, super-resistant gram-negative rods (CRE, CPE) have increased rapidly with transmission of genes with total drug resistance (NDM-1) [1, 2, 3, 4, 5]. Bacteria sensitive to common antibacterial agents will probably not be a major problem in the future, unless changed antibacterial sensitivity.
  • Anthrax is still endemic or hyperendemic on all continents in the world, mainly as a zoonosis among unvaccinated cattle [1, 2, 3, 4, 5, 6, 13, 33]. It has been known for more than 4000 years and was earlier called “Black bane”. Natural anthrax in human is associated with contact with sick animals and animal products. The bacterium Bacillus anthracis is producing lethal toxins and spores that can survive in the environments for 100 years or more. Minimum infectious dose is 1–3 spores, and toxin-producing anthrax may cause skin, lung or intestinal anthrax, dependent on the transmission via contact, air or food. The non-skin anthrax has a very high lethality. Today, about 2000 people are infected each year, from eating or handling infected meat. During peace time, there are few incidents of threats using bioterror weapons like anthrax. However, autumn 2001 anthrax was spread in the United States by “powder letters” [18, 19].

  • Plague, caused by Yersinia pestis (Enterobacteriaceae), is a zoonotic disease, vector-transmitted from infected rodents to humans via infected insects (fleas and lice) [1, 2, 3, 4, 5, 8]. It is one of the great historical diseases. In the last 1500 years, plague has killed more than 200 million people during large epidemics (Black Death), especially from the year 1347. Still, plague causes bubonic-glandular pest, lung pest and/or septicaemia in 2000–3000 cases each year, and more than 10 outbreaks (Africa, Asia, Australia, Madagascar) have occurred since the year 2000 [8, 34].

  • Cholera caused by Vibrio cholera is one of the great quarantine diseases, called “mother of all epidemics”, still very active in Asia and Africa. Cholera is associated with contaminated water, war and nature catastrophes, like in Haiti 2010, with 670,000 cholera cases (profuse diarrhoea) [1, 2, 3, 4, 5, 35].

  • Brucella species (Brucellaceae) are one of the most important zoonotic diseases both for animals and humans all over the world. They easily spread via contact and air from animals and food, survive in the environment for months especially during colder seasons, are very easily transmitted and may cause nosocomial infections, chronic febris undulans, septicaemia and lung diseases. Vaccination of animals may eradicate the zoonotic disease [15]. Brucella are particularly related to laboratory outbreaks but are easily transferable outside the laboratory and are considered highly infectious [36].

  • Francisella tularensis (zoonosis) is defined as a category A bioterrorism agent, highly infectious and increasing in the society, has low infection dose (one to ten bacteria) and can be inhaled or infected via food and water [1, 2, 3, 4, 5, 8]. Occasionally, such patients are detected in hospitals, even in the operating department [37]. Tularemia is a zoonotic disease or colonization among wild animals as rodents and hares and may survive at colder seasons for long periods in water, amoebas, birds and insects like ticks. The infection (skin, lung, intestine) is transmitted via contact with infected animals, inhalation of airborne bacteria, contaminated water and ticks.

  • Botulism is caused by a bacterial-produced toxin (Clostridium botulinum) that causes paresis and is common in soil as spores [1, 2, 3, 4, 5]. The disease can be associated with toxin formation in contaminated and poorly canned foods, shrimp fish, bacon, etc. under anaerobic conditions and randomly affects both healthy people and vulnerable groups, such as infants who have had honey infected with the bacterium, which has happened repeatedly [38]. The toxin is the most dangerous we know and is on the list of bioterrorism.

  • Family Rickettsiaceae is associated with many different types of small, intracellular microbes that are zoonotic and survive for a long time in the nature and in infected animals, birds, insects, water and amoeba [2, 3, 10]. Coxiella burnetii is a global zoonosis among animals like sheep and goats and may be airborne for several kilometres, causing frequent outbreaks of Q-fever in Europe in more than 4000 cases in the Netherlands during 2007–2010 [10]. Rickettsia species are more than 25 types, and many of them, especially the typhus group, are causing severe diseases. Humans are mostly infected by insects, including ticks from infected wild animals and rodents usually found in tropical or subtropical areas, but climatic changes, lack of good hygiene, war and natural catastrophes may spread these infections. During the period 1881–1920, 30 million people got epidemic typhus, and three million died in East Europe and Russia [2, 3, 10].

80.1.3 Other Bacteria That Are Causing Concern

  • Borrelia (Spirochetaceae) has an increasing number of species that may infect human cases all over the world. The disease may cause chronic infections in the skin, muscles, heart, nerve tissue and brain. Borreliosis is a typical emerging zoonosis, spread via different ticks from animals and birds over large geographic areas and with large climatic variations [9].

  • Syphilis (Spirochetaceae) and Treponema pallidum are old, historical diseases, transmitted via sex, from mother to child during pregnancy and via blood products. Now re-emerging and increasing worldwide [9].

  • Other spirochetes or the like may cause zoonosis, like Borrelia recurrentis and Leptospirosis [9].

  • Burkholderia pseudomallei is causing melioidosis, pneumonia, septicaemia, disseminated infection and shock. Incubation period may be up to 20–40 years. It is a long survivor in the environment (2–10 years), robust at tropical-subtropical temperatures >25 °C and is very dangerous. Endemic in Southeast Asia and may be spread by increasing globalization [1, 2, 3, 4, 5].

80.1.4 Other Infective/Dangerous Agents

  • Fungi and mould—different types are especially related to floods, water damage, etc. and to contamination of medical products and equipment [39, 40]. Especially, the newly defined Candida auris may be very virulent.

  • Prion disease-new Shy-Drager syndrome is rediscovered in 2015; Multiple-system atrophy (MSA) [5, 41].

  • Ricin is a plant-derived toxin that is still used for bioterrorism in letters to, among others, President Obama [42].

Serious and unusual microbial infections


Geographic area



Arena virus


West Africa

Severe, often fatal




Junin (Argentine haemorrhagic fever-HF)



Machupo (Bolivian haemorrhagic fever)








Mild to severe




23–100 % fatal



20–100 % fatal

Bunyaviridae (over 60 viral types from asymptomatic to fatal)

Rift Valley fever


1 % develops HF

California enceph

United States, Europe

2 % fatal

Crimean-Congo haemorrhagic fever (CCHF)

Africa, Asia, Europe


Hantaan group

North and South America, Europe, Asia

Moderate to severe

Arbovirus (Togaviridae and Flaviviridae)

Tick​-borne (TBE) encephalitis virus complex

All continents

Moderate to fatal

Russian spring and summer encephalitis (RSSE)


Omsk haemorrhagic fever


Central European encephalitis viruses


Venezuelan equine encephalitis virus


Moderate to severe

Yellow fever virus

Africa, America

Moderate to severe

Semliki Forest virus

Africa, epidemic

Mild to fatal

Dengue virus

All tropical-subtropical areas

Mild to fatal

Japanese B encephalitis

Asia; Korea, Japan, Philippines

Often fatal


In all parts of the world

100% fatal without tr.

Monkey pox (B)

Animal experiments

Severe, often fatal

Smallpox Virus

Eradicate, but kept

Severe, often fatal in unvaccinated

Avian influenza A H5N1

Asia, Africa, Europe

80% fatal without treat

Nipah encephalitis virus (paramyxovirus)

Asia, Malaysia, India, Bangladesh

75% fatal

Corona Virus

 Systemic acute respiratory syndrome (SARS)

Asia, Canada, proliferation

Severe, 10% fatal

 Middle East respiratory syndrome (MERS)

Middle East, emerging

Severe, 30–40% fatal




All continents

Many types, severe


Asia, Australia

Moderate to severe


All continents

Moderate to severe

Coxiella burnetii

All continents

Moderate to severe

Bacillus anthracis

All continents

Moderate to severe


All continents

Mild to severe

Burkholderia pseudomallei


Mild to severe

Francisella tularensis

All continents

Mild to severe

Vibrio cholera

Asia, Africa and South America

Mild to severe

Bacillus anthracis

All continents

Moderate to severe

Yersinia pestis

Africa and Asia

Moderate to severe


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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Bjørg Marit Andersen
    • 1
    • 2
  1. 1.Department of Hospital InfectionsOslo University HospitalOsloNorway
  2. 2.Faculty of Health and Social ScienceUniversity of South-Eastern NorwayOsloNorway

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