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Infectious disease

Infectious disease is an illness due to a specific infectious agent or its toxic products, that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment. Contagious disease is a slightly obsolete term, but if used nowadays it usually means 'highly infectious'.


Exposed, Infected, Susceptible

Someone who has met with an infectious agent in a way that we know from experience may cause disease has been exposed. This definition is somewhat circular, and implied that the concept of exposure relies on present biological knowledge of transmission mechanisms. Someone who passed a patient with hepatitis B infection in a corridor has not been exposed to hepatitis B virus. A person that has had sex with another person with hepatitis B has been exposed to this disease. If the infectious agent manages to get a foothold in the exposed person, he becomes infected. Infection leads to a carrier state for some diseases. A carrier harbors the pathogen -and is able to transmit it- but in this carrier state the person is not really infected with the bacteria, it is only colonized. Someone who has experienced an infection with a certain pathogen - or who has been vaccinated against it is said to be immune. Those who are not immune to a disease, and thus potentially infected by an exposure, are called susceptible.


What is a case?

Much of routine infectious disease epidemilogy relies on routine reports of noticeable disease. Using such figures, cases may be compared over time or between regions/countries. The above list of possible outcomes following an exposure makes it clear that the definition of a case is far from simple. To be registered as a case in the classical sense: 1) the patient has to experience symptoms from the infection, and be ill enough to seek medical care or advice, 2) the physician then has to suspect the correct diagnosis, and in most cases send a sample to the laboratory, 3) the test in the laboratory must come out positive, 4) the case must be reported and 5) the case has to be filed correctly at some central agency.

It is obvious that the number of cases included in the regional/national statistics will underestimate the true number of infections, to varying degrees for different diseases.



Incidence is defined as the number of people who fall ill with a certain disease during a defined time period. If this period is not stated, it is always assumed to be one year. The statement 'the incidence of tuberculosis is Buenos Aires is about 43,000', thus means that some 43,000 persons get ill born tuberculosis in Buenos Aires each year. To be able to make comparisons between regions and countries it is common to divide by total population of the area. The incidence of tuberculosis in Eastern Europe is about 48 per 100,000 inhabitants and year, whereas in Southeast Asia it is several times higher. Another way of expressing incidence is by giving the percentage of a population that will have the disease during one year. If incidence is measured over a longer time period, it is often replaced by the term cumulative incidence.



The prevalence of a disease is the number of people who have that disease at a specific time. Like the incidence, this figure is often divided by the total population of the region. A person who falls ill adds one to the incidence of the disease. He will also add one to the prevalence for the duration of his disease, either until he recovers or dies. If the average daily incidence of a disease is called I and the average duration is D days, then the average prevalence, P, will be :

P= I x D

or in words : 'prevalence is the product of incidence with duration'.

Most infectious diseases have such a rapid course that 'prevalence' becomes a rather uninteresting value


Primary/secondary cases

For infections that are spread person-to-person, the individual who brings the disease into a population (where the population can be any defined group of people, such as a school class, a group of restaurant visitors, or even a country) is called the primary case. The people infected by him/her are called secondary cases. If all the secondary cases are infected at about the same time, then the tertiary cases will also appear approximately simultaneously, and we can talk about waves, or generations, of infection.


Reproductive rate (R0)

The potential for a contagious disease to spread from person to person in a population is called reproductive rate. It depends not only on the risk of transmission in a contact, but also on how common contacts are: a person with measles who meets no-one will not transmit the infection. In a similar way the rate of acquisition of new sexual partners will influence the spread of sexually transmitted diseases. The principal determinants of the reproductive rate are:

  1. The probability of transmission in a contact between an infected and a susceptible individual.
  2. The frequency of contacts in the population.
  3. How long an infected person is infectious.
  4. The proportion already immune in the population.

The spread of infectious diseases not only depends on the properties of the pathogen or the host, but in at least equal degree on the contact patterns in the society -who meets whom?; how often?; what kind of contact do they have?


Epidemic and endemic
Epidemic :  an excess of cases from what is normally expected, or the appearence of a new infection, in the community. The point at which an endemic disease becomes epidemic depends on the usual presence of the disease and its rate. Within an unusual disease, two cases can constitute an epidemic, whereas with a common disease, an epidemic occurs when the usual rate of the disease is substantially exceeded.

 Endemic :  the continous presence of an infection in the comunity, i.e. it is the ongoing, usual level of , or constant presence of a disease within a given population or geographic area- the usual prevalence or a specific disease within a given area or group.

Exogenous reinfection

The inhalation of tubercle bacilli by individuals who had a primary tuberculosis infection in the last five years, generates a high risk of development of pulmonary tuberculosis soon after this reinfection.
Endogenous reactivation

The tubercle bacilli resulting from primary infection can remain alive within their human host  for his/her lifetime, and at any time it can start multiplying to produce the progression to pulmonary tuberculosis .

Miliary and meningeal tuberculosis
Milliary Tuberculosis: Infection involving many different organ systems, including the lungs. The name is related to the small size of lesions, which are usually <2mm in diameter, ressembling millied seeds. 8% of extra-pulmonary TB reported in 1990 in the USA was milliary, and it was more common in older age groups and in AIDS patients.

Meningeal Tuberculosis: This is an extra-pulmonary TB that occurs in the central nervous system. It is the most lethal manifestation of TB. Mortality rate may be as high as 78%.


Meta-analysis is a formal statistical technique used to combine results from similar studies testing the same hypothesis in order to increase the number or study subjects and help objective interpretation of results (Spector and Thompson 1991 *).

This analysis can provide clear qualitative conclusions about treatment policies, but precise quantitative results need to be interpreted cautiously. Selection of studies is of key importance, and biased selection criteria should be avoided whilst at the same time ensuring a degree of homogeneity between those studies included . Studies where bias and confounding have not been adequately accounted for should generally be excluded, and consideration should be given to the possibility of publication bias.

* The Potential and Limitation of Meta-Analysis. Epidemiology Community Health. 1991; 45: 89-92

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