Table of contents :

  • classification
  • transmission
  • contagion
  • host-parasite interactions
  • dynamic interactions between pathogens
  • epidemiology of infectious diseases
  • laboratory examinations
  • web resources
  • associations
  • journals
  • bibliography

  • germ theory : the doctrine that infectious diseases are of microbic origin.


    An estimated 1,415 microbes are infectious for humans. Of these, 868 (61%), are considered to be zoonotic; overall, zoonotic pathogens are twice as likely to be associated with emerging diseases (Taylor LH, Latham SM, Woolhouse ME. Risk factors for human disease emergence. Philos Trans R Soc Lond B Bio Sci. 2001;356:983–9).
    Total cases of death caused by infectious diseases in 2000 : 17 million (30% of all deaths) infectious disease : a disease caused by a pathogenic microorganism; the etiologic agent may be a bacterium, virus, fungus, or animal parasite, and may be transmitted from another host or arise from the host's own indigenous microflora notifiable or reportable disease : one required to be reported to federal, state, or local health officials when diagnosed, because of infectiousness, severity, or frequency of occurrence


    Transmission may be ...

    vection : the carrying of disease germs from an infected person to a well person. It is The incidence of diarrheal disease among cruise ship passengers declined from 29.2 cases per 100,000 passenger days in 1990 to 16.3 per 100,000 passenger days in 2000. In 2002, the Vessel Sanitation Program of the Centers for Disease Control and Prevention reported 29 outbreaks (3% or more passengers ill) of acute gastroenteritis on cruise ships, an increase from 3  in 2001. During 2001-2004, the background and outbreak-associated incidence rates of passengers with acute gastroenteritis per cruise were 25.6 and 85, respectively. Acute gastroenteritis outbreaks per 1000 cruises increased overall from 0.65 in 2001 to 5.46 in 2004; outbreaks increased from 2 in 2001 to a median of 15 per year in 2002–2004. Median ship inspection scores remained relatively constant during the study period (median 95 on a 100-point scale), and were not significantly associated with either gastroenteritis incidence rates. Despite good performance on environment health sanitation inspections by cruise ships, the expectation of passenger cases of gastroenteritis on an average 7-day cruise increased from two cases during 1990–2000 to 3 cases during the study period. This increase, likely attributable to noroviruses, highlights the inability of environmental programs to fully predict and prevent risk factors common to person-to-person and fomite spread of diseaseref. Approximately 60 episodes of viral gastroenteritis aboard ships, involving 10 000 to 11 000 passengers, have been reported since 1986 (unless otherwise indicated, outbreaks affected ships embarking from the USA) : Contagion : the communication of disease from one individual to another. It may be ...
  • direct contagion (parasites are transmitted from one individual to another belonging to the same species)
  • indirect contagion : environmental microorganisms are often more drug-resistant than parasitic ones due to daily survival from antibiosis (e.g. : down-regulated and narrowed porins)..
  • The outcome of a host-parasite interaction depends on the pathogenicity of the parasite and the relative degree of resistance or susceptibility of the host. Main steps are : Koch's postulates are a set of laws formulated by Robert Koch to prove that an organism is the causal agent of disease. Molecular Koch's postulates : The "damage-response" framework of microbial pathogenesis is based on 3 tenets : The outcome of host microorganism interactions :
    host benefit
    microorganism benefit
    ingestion of certain bacteria or fungi by amoebae (the host) can be beneficial to the amoebae, as the microorganisms are a source of food, and detrimental to the ingested microorganism
    human intestinal flora synthesizes vitamin K and the host provides a niche with nutrients
    interactions that can result in microbial pathogenesis : 
    • ingestion of Legionella pneumophila or Cryptococcus neoformans by Acanthamoeba castellanii can result in death of the host - a process striking similarities to the interaction of these microorganisms with macrophages in mammalian hosts
    establishment of microbial infection in dead-end hosts (eg. cysticercosis)
    Microbial viability is not a requirement for microbial pathogenesis : for example cysticercosis - a devastating neurological disease that is caused by the host inflammatory response to the cestode Taenia solium - can be precipitated by the death of the parasite, such that anti-helminthic therapy is considered to be detrimental in certain clinical situations.
  • Damage mechanisms in microbial pathogenesis
  • A pathogenic microorganism is a microorganism that has the capacity to cause damage at least under certain conditions. A new pathogen-classification scheme is based on damage-response curves, the y-axis represents the amount of host damage (positive values : 0 => colonization => latency => symptomatic disease => death) or host benefit (negative values : 0 => commensalism => benefit) as a function of the quantiative and qualitative degree of the host response : Therapeutic intervention can shift the damage-response curves downwards and to the left, deteriorations in host immunity can shift them upwards, and exuberant immune responses, such as those elicited by some vaccines, can shift them upwards and to the right.

    Following infection by the microorganism, 4 main outcomes, or states, are possible :

    b | a microbial factor is entirely responsible for host damage — for example, a toxin that causes damage irrespective of the host response because toxin action is so rapid and/or the amount of toxin is insufficient to trigger an immune response.
    c | the Class 4 curve is extended below the x-axis. Such a theoretical microorganism would be a commensal in the setting of intermediate host responses, but pathogenic in hosts with either weak or strong responses.
    d | the inverted parabola represents a putative host–microorganism interaction that induces damage over a narrow and limited range of responses, but not in the presence of either strong or weak host responses. One example of such a phenomenon would be an antibody response to a hypothetical microorganism, whereby host damage is caused by antigen–antibody complexes. Although we cannot think of a specific microorganism that fits this description at this time, examples of this type of host damage are the host–microorganism interactions characterized by the Jarisch-Herxheimer reaction following treatment of syphilis, the similar reaction that can occur after the initiation of therapy for Pneumocystis cariniipneumonia, and serum sickness following the injection of foreign protein.

    These states are a consequence of the outcome of the amount of host damage that results from host-microorganism interactions over time, and are generally continuous, such that when damage exceeds a threshold amount, another state becomes relevant. The use of damage as the common classification for microbial pathogenicity and virulence simplifies the lexicon of microbial pathogenesis and makes it possible to discard ambiguous terms, such as commensal, saprophyte, opportunist, exposure, and carriage.
    Host damage-time plots : e.g.


  • Dynamic interaction between pathogens
  • The processes which give rise to emerging infectious diseases (EID) of wildlife can be categorised as follows :
  • During the past 25 years there have been >30 newly-identified diseases that have spread to humans. Effective human-to-human transmission requires that the pathogen's basic reproductive number, R0, should exceed one, where R0 is the average number of secondary infections arising from one infected individual in a completely susceptible population. However, an increase in R0, even when insufficient to generate an epidemic, nonetheless increases the number of subsequently infected individuals. Here we show that, as a consequence of this, the probability of pathogen evolution to R0 > 1 and subsequent disease emergence can increase markedly.
  •  Laboratory examinations :
  • Primary prevention
  • 'rumour surveillance' : public-health experts are poised to exploit an unlikely weapon in the war against bird flu and other fatal diseases. They want to expand a worldwide system for eavesdropping on rumours. Listening to gossip may sound like a flimsy means of spotting potentially devastating microbes and viruses. But the World Health Organization (WHO) already uses 'rumour surveillance' to monitor online media for early signs of epidemics, including ebola, cholera and severe acute respiratory syndrome (SARS). Epidemiologist Gina Samaan of the WHO's Western Pacific Regional Office in Manila and her colleagues examined whether a 2004 effort to detect rumours of bird flu helped combat the disease as it whipped through poultry flocks in Asia. Of 40 rumours from websites, newspapers, e-mails and experts, 9 were found to be true, and several prompted action that may have helped to stem the disease's spreadref. For example, initial reports of duck deaths in China were later confirmed to be avian influenza, and prompted over 40 countries to ban imports of poultry from China. Traditionally, the beginnings of a disease outbreak have been picked up by medical clinics and reported to government agencies, who in turn inform the WHO. But this can be slow, because many countries lack the resources to monitor and report diseases properly and the system can get bogged down in bureaucracy and politics. The spread of electronic communication and the Internet opened up a comprehensive and speedy way of scanning the globe for disease. To take advantage of this, the WHO set up its rumour surveillance network in 1997. The mainstay of the system is a sophisticated search engine called the Global Public Health Intelligence Network, based in Canada. It continuously scans and filters all news wires and online media reports for keywords such as 'outbreak' and 'epidemic' in 6 languages, ranging from English to Arabic. A team of experts checks whether the hits are really outbreaks of international concern, based on the severity of the disease and the credibility of the source. This information is passed to WHO experts in regional offices around the world, who may ask the country involved for verification that the disease is real and then help them to tackle it. The WHO compiles its own figures on the effectiveness of its rumour surveillance. From the tens of thousands of initial reports detected between January 2001 and October 2004, the system picked up around 1,300 disease outbreaks of potential global importance : of these, 850 proved real. The WHO now wants individual countries to set up rumour surveillance systems that would run alongside the WHO's international one. These could filter reports according to different criteria that might be better at unearthing local incidents and diseases that are important to a particular country. The establishment of such national systems is increasingly urgent because of the possibility that avian flu might spread into humans and trigger a pandemic. Many countries may be pushed to introduce rumour surveillance if an updated set of rules, called the International Health Regulations, which are likely to be adopted at the WHO's World Health Assembly in May, comes into effect later in 2005 requiring member countries to carry out real-time surveillance for a plethora of diseases and report potential emergencies to the rest of the world.
  • biocontainment patient care unit (BPCU)ref
  • Web resources
  • Associations (see also Italian associations)
  • Journals
  • Bibliography

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