Microbes are a constant source of surprise. Their «invisibility» does nothing but increase our puzzlement when a microorganism, or its activity, become evident for the first time. Although microbes are very special organisms, and therefore interesting in and of themselves, they are also interesting because they affect all aspects of existence for humans and any other living being with their harmful and/or positive effects. Unfortunately, we tend to remember the negative ones more easily. Infectious diseases have accompanied us throughout history and have had a decisive impact on many nations and civilizations. Especially those which were previously unknown, such as smallpox in the Americas or syphilis in Europe during the first half of the sixteenth century.
The World Health Organization (WHO) estimates that some two thousand organisms are human pathogens (including prions, viruses, bacteria, protists and fungi, plus some worms and arthropods), which is a small amount in the collection of millions of species of microorganisms, known or yet to be discovered. In the words of North American microbiologist John L. Ingraham, «the number of pathogenic bacteria, compared to the total number of bacteria, is much lower than the frequency of serial murderers in the human population».
The last third of the nineteenth century brought knowledge of both the culture and identification of most bacterial pathogens. Subsequently, the development of vaccines and antibiotics has contributed decisively to the fight against disease. The application of general control measures such as water sanitation, personal hygiene, public health education and improved nutrition has also been essential. However, between 1940 and 2010, more than 300 emerging diseases that affect human health have been identified around the world. Emerging infectious diseases include, first, pathogens that become new strains within the same species, such as antibiotic resistant strains (e.g., methicillin-resistant Staphylococcus aureus, MRSA). Second, pathogens that suddenly change hosts, like the human immunodeficiency virus (HIV/AIDS) or severe acute respiratory syndrome (SARS). In some cases, this change to a new host species depends on geographical changes of the pathogen (e.g., West Nile virus in the Americas). And thirdly, zoonotic diseases as a result of changes in land use, in food production practices – agricultural or otherwise – or wild game hunting (Ebola virus). These human activities increase the frequency of contact between humans and animals, and consequently with their microorganisms. We cannot forget the exponential increase in international travel and trade, which allows for the globalisation of diseases that were once limited to certain geographic areas.
The loss of species in a given ecosystem can have serious consequences on the distribution and incidence of infections, including those that affect humans. In general, it is believed that the loss of biodiversity increases the transmission of infectious diseases, especially to unusual hosts. Although the reason why that happens is not known exactly, it seems that the decrease in the number of species may increase the probability of encounters between the pathogen microorganism and the usual susceptible host. The increased incidence of infection of the West Nile virus in the United States is a good example of this. The virus is transmitted through mosquito bites and maintained among bird populations. Occasionally, it can infect a person and cause a severe type of encephalitis. In areas where the diversity of birds has decreased and species susceptible to this virus predominate, the amount of mosquitoes and infected humans increases. By contrast, in areas with more bird species, more animals are not good hosts for the virus and/or the virus cannot multiply effectively.
There are two basic concepts that must be kept in mind: mortality and morbidity. Mortality is the percentage of individuals who die of the total affected by the disease. Morbidity is the proportion of people who are affected by the disease within the considered population. Therefore, we could say that the cold or flu viruses have very high morbidity but very low mortality, whereas the Ebola or Marburg viruses are the opposite. The Ebola and Marburg haemorrhagic fevers are caused by virus of the Filoviridae family (negative-polarity single stranded RNA virus). These are high-mortality, low-morbidity viruses. The Marburg virus was named after the German town where it was first isolated in 1967 in infected researchers of a centre where they worked with samples from Ugandan monkeys. The Ebola virus is named after the Ebola River (Democratic Republic of Congo, formerly Zaire), where it was first identified in 1976. Bats are believed to be the natural reservoir of the Ebola virus, and sub-Saharan Africa is their endemic area. The fact that outbreaks seldom occur suggests the presence of a rare or ecologically isolated animal reservoir, which has little contact with humans and other primates. Recently, a new filovirus has been detected in bats in the Iberian Peninsula, Lloviuvirus, which is genetically different from Ebola and Marburg, but its pathogenicity in humans has not yet been observed. Against these filoviruses, there are no vaccines or effective treatment therapies today.
The origin of the disease in people infected with Ebola is mainly related to exposure to animal carcasses found in the forest or by direct contact with bats, and the subsequent transmission of the virus through contact person to person. The virus enters the body via contact or infected bodily fluids (blood, saliva, sweat, vomiting, etc.). After an incubation period that goes from two to 21 days, the initial nonspecific symptoms such as fever, chills, fatigue, headache and myalgia appear. In fatal cases – rather common – patients die of hypovolemic shock and multiple organ failure between the sixth and sixteenth days.
The human disease is linked to the frequency of contact with infected primates, but other animals are also involved. It seems that the population of gorillas and chimpanzees has fallen by 80% in some parts of Central Africa due to Ebola virus infections.
Factors such as climate change, environmental destruction, increased population, mass migration and air travel make infectious diseases previously geographically confined in specific locations to spread quickly around the globe, often in time spans that are shorter than the incubation periods of the disease. The struggle between pathogens and host goes on and it increasingly requires scientific research to face it. We must have effective tools for the detection, diagnosis and treatment. Infectious diseases are quite common and widespread. There is no specific age to suffer from them, although the high-risk groups are children and the elderly. The situation and development of current microbiology allows us to foretell the presence of this science as a major weapon in the fight against death throughout the twenty-first century.
«The loss of species in a given ecosystem can have serious consequences on the distribution and incidence of infections, including those that affect humans»