In this session, we will discuss some basic concepts in infectious disease epidemiology. Epidemiology is the basic science of public health. In epidemiology, we study the patterns and determinants of diseases in the population. For example, where and when does the disease of interest appear? What is the burden of the disease in the population? Do people who have the disease have any common characteristics that are not present among people who do not have the disease? As public health epidemiologists, we aim to understand the epidemiology of diseases for improving preventive and control measures against them. Infectious diseases are diseases caused by pathogens including viruses, bacteria, and parasites. The most important difference between infectious and non-infectious diseases is that most infectious diseases are communicable, which means that these diseases can be transmitted from person to person, either directly, for example, via droplet or airborne transmission for influenza, or indirectly, for example, via mosquito bites for malaria. The risk of getting a communicable disease increases with the number of people who have the disease in the population, a feature that is absent for non-infectious diseases. For example, the risk of a child getting influenza is greatly increased if his classmate sitting next to him has influenza. However, the risk of a child getting diabetes does not directly depend on whether his classmate has diabetes. Another important difference between infectious and non-infectious diseases is that for some infectious diseases such as measles, a recovered person gains long-lasting immunity against re-infection by the same pathogen. We say that an individual is susceptible if he is neither infected nor immune, and therefore can potentially be infected if he encounters the pathogen. Obviously, more immune people means fewer susceptible people. The spread of a communicable disease is limited by the proportion of population that are immune, the so-called herd immunity effect which we will learn later this week. By the same token, the persistence of a communicable disease in a population relies on the replenishment of susceptibles. For example, via newborns, immigration of susceptibles, or waning of immunity. In epidemiology, we say that a person has been exposed to a pathogen if the person has encountered the pathogen in a way that the person could become infected. For example, a child who has been sitting next to another child infected with influenza for an hour has been exposed to influenza. Exposure does not necessarily lead to infection. In our example, the child may not be infected after being exposed for an hour. If exposure leads to infection, the infected person may develop symptoms in which case the infection is clinical. For many diseases, such as influenza and polio, the infection may be subclinical or asymptomatic, which means that the infected person does not have any clinical symptoms over the course of the infection. Clinical infection may result in death or recovery with or without immunity. For example, almost all measles infections are clinical with long-lasting immunity after recovery. Similarly, subclinical infections result in recovery with or without immunity. Epidemiologic study of an infectious disease starts with the case definition of the disease. A consensus on case definition is essential for comparing disease patterns across time and geographical areas. The case definition of a disease could be based on laboratory results, clinical symptoms as well as epidemiologic links. For example, during the SARS epidemic in 2003, the WHO definition for a suspected case of SARS had a clinical and an epidemiologic component. The clinical criterion was either a person presenting after 1 November 2002 with history of high fever and cough or breathing difficulty, or a person with an unexplained acute respiratory illness resulting in death after 1 November 2002, but on whom no autopsy has been performed. The epidemiologic criterion was one or more of the following exposures during the 10 days prior to onset of symptoms. First, close contact with a person who is a suspected or probable case of SARS, or two, history of travel to an area with recent local transmission of SARS, or three, residing in an area with recent local transmission of SARS. With a clear case definition, we can then begin to describe disease burden by tracking the occurrence of cases. Incidence is defined as the number of cases that occur during a defined time period, divided by the population size. The time period can be days, weeks, months or years. For example, in 2012, there were 86 cases of AIDS in Hong Kong which had a population size of around 7.2 million. So the incidence of AIDS in Hong Kong in 2012 was about 1.2 per 100,000. Incidence is often calculated from clinical cases seeking medical care, in which case the true number of infections would be underestimated if (i), a substantial proportion of infections are subclinical or (ii) a significant proportion of clinical cases do not seek medical care and therefore not observed by the healthcare system. For example, a UK study recently reported that among their subjects, around 3/4 of influenza infections were subclinical and that only 4% of all influenza infections were medically attended. Prevalence is another useful measure for disease burden. It is defined as the total number of cases at a specific time, divided by the population size. For chronic or protracted infections, prevalence is the product of incidence and average disease duration. Most infectious disease epidemiologic studies are observational science, which means that we cannot perform reproducible, controlled experiments as we could in lab settings for physics and chemistry. This is because it is impossible to set up precisely the same set of conditions to generate the same exact epidemics. Furthermore, deliberate release of pathogens in human populations for scientific purposes is medically unethical. Our best alternative is therefore to study the cause and spread of diseases by analyzing the data collected from naturally occurring epidemics. As such, robust disease surveillance and meticulous medical records are essential for our understanding of and combat against epidemics. To summarize, in this session, we have discussed some basic concepts in infectious disease epidemiology.
