The Epidemiology of Infectious Diseases

Infectious diseases have always existed and have had a major impact on human development. It is widely believed that our immune systems and genetic makeup have evolved over many years under the selective pressure of potentially fatal diseases, such as malaria (Haldane 1948; Weatherall 1996).
In addition, epidemics of infectious diseases have decimated entire communities, and have sometimes changed the course of history.
Examples

In Europe in the 14th Century, there were about 25 million deaths from bubonic plague out of a population of approximately 100 million.
In 1520 the Aztecs lost about half of their population of 3.5 million from smallpox, introduced by the more immune invading Spaniards. This has been proposed as an important feature in the defeat of the Aztecs by the Spanish invaders.
In 1919, after the First World War, the global epidemics of influenza killed an estimated 20 million people during one year – more than died as a result of the war.

During the 20th century, important advances in the prevention and control of many infectious diseases were achieved with the development of vaccines and antibiotic drugs. This has sometimes created the impression that infectious diseases are no longer a major threat to public health. However, this is far from being the case.
The following is a quote from Dr Gro Harlem Brundtland, the Director-General of the World Health Organization:
“Illness and death from infectious diseases can be, in most cases, avoided at an affordable cost. It is in everyone’s interest that these obstacles to development be removed. Because of drug resistance, increased travel and the emergence of new diseases, we may only have a limited time in which to make rapid progress.”
In the following, you will see some of the reasons why infectious diseases are still an important challenge to public health at the beginning of the 21st century.
Mortality
Infectious diseases are a leading cause of global mortality, causing more than 13 million deaths a year.
They are still the main cause of death among children under 5 and the main single cause of premature death in persons under the age of 45.
Morbidity
Infectious diseases are also a major cause of global morbidity. They are responsible for a huge amount of disability and suffering in the world as measured in DALYs.
DALY
Disability Adjusted Life Years, a measure of disease burden.
It includes years of life lost due to premature death, and years of healthy life lost due disability or illness.
Recurring episodes of illness and long-term disability have a major economic impact on the developing countries most affected by infectious diseases.
Role in chronic disease
Infectious diseases are increasingly being implicated in the pathogenesis of many important diseases that were previously thought to have a non-infectious origin.
Cervical cancer is now known to be associated with human papillomavirus infection. Cervical cancer is the sixth most common cancer worldwide and the most common cancer in women in many developing countries.
In the past two decades, evidence has grown on the role of Helicobacter pylori infection in gastritis, duodenal ulcer, gastric ulcer, gastric cancer and gastric-mucosa-associated lymphoid tissue (MALT) lymphoma.
Chronic infection with hepatitis B or C can cause primary hepatocellular carcinoma (HCC). HCC is among the most common cancers in many parts of Africa and Asia.
Potential for epidemic spread
A specific feature of infectious diseases is their ability to be transmitted between individuals. This can result in the occurrence of large outbreaks.
Between 1997-2000 there were more than 600 outbreaks of disease considered by the WHO to be of ‘international importance’.
Outbreak
The term used to describe a localised epidemic, e.g. in a village, town or city.
The term ‘large outbreak’ is increasingly being used instead of ‘epidemic’, as it is less emotive.
With increasing urbanisation and international travel, the world is becoming a smaller place, and the routes for transmission of infection are increasing. Aeroplane journeys enable individuals to travel within the incubation period of most infectious diseases.
This allows infections to spread to distant places within very short periods of time. An example of this is the annual global dispersal of meningococcal meningitis by pilgrims returning from the Haj Muslim religious festival (Saudi Arabia).
Newly emerging diseases
Over the past three decades, over 30 new infectious diseases and pathogens have been identified for the first time in humans.
These include diseases with a very high case-fatality rate, such as new variant Creutzfeldt-Jacob disease (nvCJD) and Ebola haemorrhagic fever. Some of these new infections are highly prevalent, for example Hepatitis C and rotavirus. Other infections, such as HIV, have rapidly spread around the world.
New variant Creutzfeldt-Jacob disease (nvCJD)
A new variant of Creutzfeldt-Jakob disease was described in the United Kingdom in 1996. The agent is considered to be the same as that causing bovine spongiform encephalopathy, a disease that emerged in the 1980s and affected thousands of cattle in the United Kingdom and other, mainly European countries.
Ebola
The first outbreaks of Ebola haemorrhagic fever occurred in 1976 and the discovery of the virus was reported in 1977. Cases reported to WHO up to June 1997 indicated a case-fatality rate of over 70%. A major outbreak in Uganda in 2000 was thought to be associated with spread of the virus by soldiers moving across the country.
Hepatitis C
This virus was identified in 1989, and is now known to be the most common cause of post-transfusion hepatitis worldwide. So far, up to 3% of the world population are estimated to be infected, among whom 170 million are chronic carriers at risk of developing liver cirrhosis and/or liver cancer.
Rotavirus
First recognised in 1973, rotavirus is the most common cause of childhood diarrhoea worldwide. 20% of all diarrhoeal deaths and 5% of all deaths in under-5 year olds are due to rotavirus.
HIV
Although the Acquired Immune Deficiency Syndrome (AIDS) was recognised in 1981, the causal virus, HIV, was first isolated in 1983. It is estimated that, since the start of the epidemic, 30.6 million people worldwide have become HIV-infected and nearly 12 million have died from AIDS or AIDS-related diseases.
Re-emerging diseases
In addition to the emergence of new infectious diseases, many old diseases that had previously been under control are starting to appear in increased numbers or in previously unaffected populations.
Resurgence of infectious diseases can occur because of any of the following reasons:

changes in social or environmental conditions,
failure to maintain immunisation programmes,
increased drug resistance

Drug resistance is currently an increasing problem for a number of diseases worldwide, and we are often in a race to develop new treatments faster than the pathogens can develop resistance.
Example
Since the mid-1980s there has been a major resurgence of diphtheria in several countries of Eastern Europe, which had previously been progressing towards elimination of the disease. In 1993, 15,211 diphtheria cases were reported in Russia and 2,987 cases in Ukraine. The main reason for the return of diphtheria in these countries was a decreased immunisation coverage due to an irregular supply of vaccines and large-scale population movements
(Galazka et al 1995).
Example
Mortality and morbidity rates from tuberculosis (TB) in industrialised countries declined during most of the 20th century. However, from the mid-1980s onwards, many of these countries have seen an important increase in the incidence of TB. This is mainly due to a decline in TB control programmes, the increased incidence of multi-drug resistance TB and the effect of the HIV epidemic (Grange 1998).
Potential for prevention and control
The mechanisms involved in many infectious diseases are well understood, from the molecular aspects of the infectious agent to the demographic characteristics of host populations. This level of understanding has enabled potentially very effective prevention and control measures to be developed for some infectious diseases.
With efficient intervention strategies and the advent of national public health agencies, elimination of specific infectious diseases has become feasible. In some cases, there has even been the possibility (or reality) of global eradication.
Following the successful WHO programme for the global eradication of smallpox through vaccination, the last naturally acquired case of this disease occurred in October 1977 in Somalia.
The countries of the Western Hemisphere have set a target for the elimination of measles by the end of the year 2005.
Polio and guineaworm are now also on the verge of eradication, after intensive, globally co-ordinated programmes. Strategies to immunise millions of children on the same day have resulted in few countries now reporting cases due to wild poliovirus.
Infectious disease epidemiology
In epidemiology, we are interested in describing and explaining the distribution of diseases in populations. The distribution of an infectious disease depends on the transmission of the infectious agent within the host population.
This is a dynamic process, which is influenced by characteristics of the specific infectious agent, characteristics of the host population and characteristics of the relationship between the infectious agent and the host.