Epidemiology is ‘the study of the occurrence, distribution and control of disease in populations. The risk of infection is not just dependent upon an individual’s susceptibility but on the level of disease within the population, the degree of population mixing and herd immunity, as well as specific features such as the communicable period, route and ease (infectiousness) of transmission.
Route of transmission
For an infectious agent to persist within a population there must be a cycle of transmission
from a contaminated source, through a portal of entry, into a susceptible host and on again.
• Direct transmission, the most common and important route, involves all forms of physical contact between humans, including sexual transmission, faecal-oral spread, and direct respiratory spread via large droplets.
• Vector-borne transmission is mediated by arthropods or insects; it is mechanical if the vector is simply a source of contamination, but biological if it is necessary for the multiplication or maturation of the infectious agent.
• Vehicle-borne transmission describes the spread from all contaminated inanimate
objects. Vehicles include clothing, food, water, surgical instruments and also biological substances such as blood and tissues.
• Airborne transmission is mediated by aerosols suspended in the air for long periods.
• A zoonosis is any infection spread from a vertebrate animal to a human.
Disease in the population
Prevalence is ‘the number of cases of infection per unit of population at a single point in time’. Incidence refers to ‘the number of new cases of infection per unit of population over a specified period of time’. For acute infections, lasting only a few days or possibly weeks, the incidence may be very high but the prevalence relatively low. However, for chronic infections, lasting months or years, the prevalence may be relatively high even though the incidence is low. Infections like urinary tract infections occur at roughly steady levels throughout the year, although others may vary – for example the rise in respiratory tract
infections during the winter. The periodicity of some infections is measured over a much longer scale, for example the roughly 4-yearly cycle in mycoplasma pneumonias.
Infections that have a stable incidence within the population are described as endemic (or hyperendemic if the incidence is extremely high). Cases are frequently unconnected and are therefore referred to as sporadic. A number of terms are used to describe situations in which the number of infections is greater than that which might be anticipated from previous experience:
• A cluster of cases in a single household or over a small area is described as an outbreak.
It may relate to exposure to a local source, and suitable detective work and control procedures may prevent further transmission.
• An increase in cases over a larger region, perhaps an entire country, is described as an
epidemic. This is less likely to be due to a single source, and more extensive measures will be required to control the spread.
• If the increase occurs over a larger area still, for example several countries, it is described as a pandemic.
The burden of infectious diseases varies enormously throughout the world , dependent upon factors such as environmental conditions, wealth and nutritional status, local human behaviour and the efficiency of health care. Health service managers clearly need this information to plan for the future; however, it is vital that every clinician be aware of regional patterns of disease for the effective diagnosis, treatment and control of infection
within their own practice of medicine. For serious communicable diseases in the UK, surveillance is achieved through a legal requirement for medical practitioners to inform
the local ‘consultant in communicable disease control’ (or the ‘consultant in public health
medicine’ in Scotland) of all cases of notifiable disease . For less serious
infections, individual general practices or hospital units may volunteer to report their cases
of infection through government or professional association programmes.