The history of mankind is regularly punctuated with adverse dealings with microbes, both good and bad: 
the great famine in Ireland; the consequences of smallpox on the native peoples of North and South 
America; the influenza pandemics and the march of AIDS across the world. To help explain the variety of different human infections, an understanding of their epidemiology is essential. Most people learn to avoid dangerous situations. Most try to avoid sitting next to someone sneezing and blowing their nose. This behaviour is based on the recognition that germs are present. All infections arise as organisms are transferred from a reservoir to a source and then transmitted to a susceptible host. The transmission of microbes between humans is critically important in the successful spread of the organism within a population. The often-called chain of infection is as follows:
                               Reservoir: Source: Mode of transmission: Host
The reservoir is the primary habitat of the organism. The differing habitats illustrate the variety of 
reservoirs, e.g. in humans Salmonella typhi, measles virus are the only known reservoirs of the organism 
(hence they are strict human pathogens). Animal reservoirs exist for all zoonoses, e.g. rabies virus, 
Salmonella enteritidis. Environmental reservoirs include soil, maintaining Clostridium botulinum, and 
water from which Legionella spp.are acquired. 
The source of infection is the actual site from where the infection is acquired. The source is not 
necessarily the same as the reservoir. Salmonellae are commonly found in the intestinal tracts of 
chickens. When you develop food poisoning, the meal in the restaurant is the source. When the infection 
is transmitted from an inanimate object it is often called the ‘vehicle of infection’ but the chicken flock 
is the reservoir. The contaminated chickens can be distributed to more than one restaurant. When an 
infection is acquired via an inanimate (non-human) source such as water or food it is often termed the 
‘vehicle’. With infections that develop from the patient’s own flora, e.g. urinary tract infections with 
Escherichia coli, an organism that normally colonize the intestine, the host is both the reservoir and the 
source! Certain infectious diseases of humans are restricted to humans only. No other animal or environmental reservoir appears to exist. It is clear that a number of human infections have no other reservoir or host than humans themselves. It also is worth noting that fungi that are strict human pathogens are not well represented. Most fungi that cause infections in humans have a free-living equivalent in the environment, such as soil or plants. Candida albicans is often found as part of the normal flora in humans. The fungi that cause infections called tinea (or ringworm) are thought to be those that have lost the ability to reproduce in the environment but have become strict parasites of humans. In the absence of finding the equivalent sexual form (the teleomorph, able to exist in the environment).  

Acute infections will be those that arise quickly (e.g. tonsillitis) and progress rapidly, in contrast to chronic infections which have a longer course (e.g. tuberculosis), lasting for weeks up to years without resolution. Latent infections are those in which the microbe is able to persist for years within a site in 
the host and cause minimal clinical disease for most of the time the organism is present. Examples of 
latent infections are Herpes simplex virus infections that result in periodic episodes of cold sores, but, in 
the absence of suitable triggers, remain silent most of the time. The virus resides within the nerve cells that supply the lips. The virus cannot be recovered from these infected cells but is able to 
maintain its genome within the cell. Varicella zoster virus, the etiological agent of chicken pox, also a 
herpes virus, is likewise able to reside in nerve cells for years until it is reactivated to cause shingles 
Other terms are useful in the description of types of infections. The extent to which an infection spreads 
throughout the body can be described by the terms localised or generalised infections. It is easy to see 
that generalised infections are often more severe. The involvement of numerous organs throughout the 
body will lead to more complications than simply infecting a site on the skin. Another term for systemic 
infection is ‘multiorgan infection’. Generalised infections will arise from infections that enter the 
bloodstream and will be distributed around the body in the bloodstream by the host. Spread through the 
body is not, however, limited to passage in the bloodstream. Streptococcus pyogenes can cause a 
cellulitis where the organism spreads through the skin. In this particular instance the movement of the 
organism through the tissue can be surprisingly quick. Spreading cellulitis can be fatal within hours if 
left unchecked. It does not follow that a localised infection is going to pose no significant threat because it is contained to one focal point. Several infections due to toxin-producing bacteria result in serious life-threatening infections. Diphtheria is a localised infection of the throat but the release of the diphtheria toxin into the surrounding tissues spreads throughout the body. The toxin is thought to be responsible for the cardiac changes, the paralysis and general toxicity As mentioned above, a distinction can be made between primary and secondary pathogens. A primary pathogen is a micro-organism that can establish itself in a new healthy host and cause disease unaided. A secondary pathogen is one that requires a breach in the normal host defences before it can proliferate in 
the host. Secondary pathogens are commonly referred to as ‘opportunists’ but this term can equally 
apply to both primary and secondary pathogens. In humans, secondary pathogens usually infect people 
with illness or other predisposing conditions that allow the organisms to gain a foothold and colonise the 
site. Salmonella typhi, the aetiological agent of typhoid is a primary human pathogen; you simply need 
to ingest sufficient numbers. Whereas, with the secondary pathogen Pseudomonas aeruginosa, there will 
need to be a breach in the host defences for the organism to establish an infection, e.g. burn injury to the 
skin, unchanged contact lens use or diabetic ulcer on the foot, before the organism can establish an infection. Staphylococcus aureus is able to cause a disarmingly diverse list of infections, yet others are limited to a particular infection. In general, those organisms that can establish themselves and 
cause illness (primary pathogens) tend to cause one particular disease. The pathogens of typhoid 
reflects the particular sequence of events that occurs as Salmonella typhi progresses through the body in 
a set pattern. Organisms like Staphylococcus aureus and Pseudomonas aeruginosa are found in a wider 
range of infections because they are able to exploit breaches in the host defence (hence the term 
‘opportunist’). Instead of following a particular route through the body like Salmonella typhi, 
Staphylococcus aureus will multiply at most body sites once given the opportunity: a breach in the skin 
(cuts, abrasions, surgical wounds) or invading the lungs to cause pneumonia following an infection with 
the influenza virus. The greater diversity in infections reflects the increasing number of opportunities in 
the patient as a result of modern medicine.

 Some diseases result in typical features that are termed pathognomic: the features are characteristic of a 
particular infection or disease. The lock jaw characteristic of tetanus or the rash of measles are 
pathognomic signs. Tetanus and measles are two infections that are caused by a particular organism. No 
other organism is responsible for measles except the measles virus. These are specific infections, with 
characteristic signs, symptoms and cellular pathology. Robert Koch demonstrated that the bacterium 
Mycobacterium tuberculosis was responsible for a specific human disease (tuberculosis) and, with the 
work of Louis Pasteur, the microbiological revolution had begun. Numerous diseases were found to 
have bacterial causes. From the list of diseases caused by single organisms (tetanus, botulism, 
tuberculosis, anthrax, rabies …) it is understandable that microbiologists believed the ‘one germ, one 
disease’ principle must be the only model. It was felt that infections are caused by a single species, and 
if more than one species was cultured from the infected tissues then it was contaminated. This idea 
stems from Koch’s postulates in which the organism must be found in the diseased tissues in pure 
culture. It is now recognised that infections may be poly microbial in origin (dental caries, anaerobic abscesses), although there are not that many.
Signs and symptoms :Medical doctors distinguish between signs and symptoms. Signs are 
features observable by the medic in the patient whereas symptoms are what the patient feels 
and complains of. A sore throat is a symptom and the reddening throat and enlarged lymph glands seen on examination are signs. 
  Most bacteria that cause clinical disease in humans are able to cause the host to respond with rapid accumulations of white cells and other inflammatory cells and compounds. This accumulation of 
immune cells, bacteria and necrotic tissue is what everyone recognizes as pus. Bacteria able to induce 
pus formation are called pyogenic, a term not to be confused with pyrogenic, the ability of certain 
organisms to induce a fever.  
Micro-organisms can infect any tissue in the body. Bacteria can infect most. Viruses will only infect 
living tissues and fungi are particularly good at extracting nutrients out of dead tissue (surface skin and 
nails and hair). It is of value to memorize a few examples of microbial infections of the different sites 
because they highlight the different paths taken by microbes in the course of an infection. The diagnostic 
microbiologist needs to be alert to the likely causes of infections in particular tissues and organs.  Note how the site of infection leads directly to that organ and the organ then subsequently develops pathology. The exception is with meningitis. The central nervous system is not directly infected by either Herpes simplex virus or Neisseria meningitidis; rather, they are acquired from respiratory secretions (saliva, kissing, etc.) and infection of the meninges is an unusual consequence following bloodstream invasion or direct entry through the nerves. This is a dead-end in terms of  transmitting the organisms to new hosts. In other infections, organisms might infect more than one organ at once. Alternatively many viral infections will show lesions on the skin but these are consequent to blood-borne spread through the body and multiplication in several  body organs.

Msc Microbiology, TU.
Assistant Professor In Pokhara University, Pokhara Bigyan Tatha Prabidhi Campus(PBPC), PNC, LA, NA.
Pokhara, Nepal.






Popular posts from this blog

Stains/ Dyes

Contributions Of Antony Van Leeuwenhoek & Louis Pasteur