Variations in Severity of Illness
The severity of an illness may be measured by the case fatality rate or the proportion of surviving patients with complications. The case fatality rate is defined as the number of deaths from a particular disease divided by the number of clinically apparent cases of that disease.
An infectious disease may have a wide variety of clinical symptoms, ranging from no symptoms to severe clinical illness or death. Diseases such as tuberculosis have a high proportion of asymptomatic individuals (low pathogenicity), while diseases such as measles have a high proportion of symptomatic infections and a small percent of severe or fatal illness. Diseases such as the African hemorrhagic fevers caused by Marburg and Ebola virus are very severe and usually fatal. For diseases with low pathogenicity, only a small fraction of cases are often diagnosed and reported. Control measures should be directed toward all infections capable of being transmitted to others, not just the symptomatic cases.
From a public health perspective, diseases of high incidence and lesser severity may be considered a more serious problem because it causes a large degree of mortality in the population as a whole.
Components of the Infectious disease process
Many factors affect the infectious disease process. Examples of factors which are dependent on the agent, include growth requirements, the ability to survive outside a host, and the ability to become resistant to antibiotics. Many factors are dependent on the interaction between the agent and the host. These factors include infectivity, pathogenicity, virulence, and immunogenicity. These factors may be affected by environmental conditions, dose, route of infection, and host factors (e.g. age, race, and nutritional status). Infectivity is defined as the ability of the agent to invade and multiply (produce infection in a host). An example of a disease with high infectivity would be measles; a disease with low infectivity would be leprosy. Techniques for evaluating infectivity include speed that an agent spreads through a population and proportion of close contacts who become infected. Pathogenicity, discussed earlier, is defined as the ability to produce clinically apparent illness. Virulenceis defined as the proportion of clinical cases resulting in severe clinical manifestations. The case fatality rate is commonly used to measure virulence. Immunogenicity is defined as the infection’s ability to produce specific immunity. Immunogenicity can be affected by host factors such as age, nutrition, dose, and virulence of infection.
An infectious agent may produce disease using a variety of mechanisms. These mechanisms include direct tissue invasion, production of a toxin, allergic reaction leading to damage in a host, chronic infection, increased susceptibility of host to nontoxic drugs, and immune suppression. Examples of pathogens that produce disease by direct tissue invasion include many parasitic diseases and viral infections. Pathogens that cause illness by producing a toxin include tetanus and Staphylococcal aureus, which occurs in staphylococcal food poisoning. Pathogens that have immunologic mechanisms include tuberculosis and dengue hemorrhagic fever. Chronic infections may occur after a clinical infection in the pharynx (e.g. Haemophilus influenzae), gall bladder (Salmonella typhi), gastrointestinal system (e.g. many species of salmonella), or the urinary tract (e.g. E. coli). An agent may also increase a host’s sensitivity to nontoxic drugs (e.g. Reye’s syndrome). Finally, HIV is an example of an infectious agent that produces disease by suppressing immunity.
Reservoirs are defined as the living organisms or inanimate matter (e.g. soil) where an infectious agent lives and multiplies. The concept of the reservoir is important in infectious disease because the reservoir is the component of the cycle where an infectious agent can survive indefinitely. Humans are the main reservoir for most of the viral and bacterial respiratory diseases. Vertebrate animals are reservoirs for diseases such as brucellosis (from cows, pigs, and goats), anthrax (from sheep), leptospirosis (from rodents), and rabies (from dogs, bats, and other animals). These diseases that are acquired from animals are known as zoonoses.
Infection has occurred when infectious agent has entered and established itself in a host. At the minimal level, the agent may be present on surface of body and multiply at a rate sufficient to maintain its numbers without producing identifiable reaction in a host, which is referred as colonization (e.g. Staphylococcal aureus in the nasal passage). At the next level, organisms multiply and cause a measurable reaction that is not clinically detectable, which is referred to as inapparent infection. The final level is when infection leads to clinical disease. All are potential sources of infection to others.
A carrier is defined as an infected person who does not have apparent clinical disease, but is a potential source of infection to others. Carriers include individuals who are asymptomatic throughout their infection, as well as those who are carriers during the incubation period or who become carriers after their symptoms subside. When the carrier state persists for a long time, the person is referred to as a chronic carrier.
Mechanism of Disease Transmission
The method by which which an infectious agent escapes a reservoir and enters a host is referred to as mechanism of transmission. There are two main types of transmission: direct transmission and indirect transmission. In direct transmission, an infectious agent is immediately transferred from one infected host or reservoir to another. Direct transmission includes not only direct contact, such as kissing, but also spray by droplets through sneezing and coughing onto the mucous membranes of others. Droplet spread is classified as direct transmission because it occurs over short distances–the droplets travel a few feet before falling to the ground.
There are three types of indirect transmission: vehicleborne, vectorborne, and airborne. Vehicleborne transmission is indirect contact through inanimate objects, such as bedding, toys, surgical instruments, and contaminated food. In vectorborne transmission, the infectious agent is transmitted by an intermediary (usually an insect) to a susceptible host. In airborne transmission, two types of particles may be spread through the air–dusts and droplet nuclei. Dusts are particles of varying size that result from resuspension of particles that have settled on floors or bedding as well as particles blown by the wind. Coccidioidomycosis is an example of a disease that is spread by airborne transmission of fungal spores. Droplet nuclei are very tiny particles that may be suspended in the air for long periods of time. They represent the dried residue of droplets from coughing, sneezing, or the aerosolization of infective materials.
Control measures may be categorized as those directed against the reservoir, those that interrupt transmission, and those that reduce the susceptibility of the host. When the reservoir involves a domestic animal, control measures may involve immunization, testing of herds, and destruction of infected animals. Control measures targeted towards human reservoirs include isolation of infected persons, temporary removal of individuals from sensitive occupations (e.g. foodhandlers with diarrheal illness), treatment to make them non-infectious, health education regarding hygiene, and disinfection of contaminated objects. Isolation refers to the separation of infected persons from those not infected for the period that they are infectious. Another control measure, related to isolation is quarantine, which is the limitation of the freedom of movement of apparently well persons or animals who have been exposed to a case of infectious disease. Control measures that interrupt the transmission of organisms include environmental measures, such as water purification and pasteurization of milk. Control measures that reduce host susceptibility include active immunization, which causes a host to produce antibodies, and passive immunization. An example of active immunization is DTP (immunization against diptheria, tetanus, and pertussis). An example of passive immunization includes the administration of immune serum globulin to prevent hepatitis A.
U.S. Department of Health and Human Services CDC (2007) Principles of Epidemiology in Public Health Practice. Third Edition. Retrieved on September 2, 2011 from http://www.cdc.gov/training/products/ss1000/ss1000-ol.pdf
Rockett I. Population and health: An introduction to Epidemiology. Second Edition Vol 54, No 4. (2002) PAHO. Retireved September 2, 2011 from:http://www.prb.org/Source/54.4PopHealthEpidemiology.pdf
Variations in Severity of Illness