Sources and communicability of infectious diseases

The spread of diseases is essential to all medical personnel, whether their work is with the individual patient or with the community. Most infections must be evaluated in their epidemiological settings.

For example, has the patient recently traveled to an area of special disease prevalent? Is there a possibility of nosocomial infection ( infection that you acquired as you enter the hospital doors either ad a visitor or as an admitted patient) from recent hospitalization? What is the risk to the patient’s family, schoolmates, and work or social contacts?

Infectious diseases of humans may be caused by exclusively human pathogens/ microorganisms, such as the Measles viruses, by environmental microorganisms such as Legionella pneumophila,or by microorganisms that have their primary reservoir in animals such as the plague bacillus.

They can generally be classified as noncommunicable or communicable.

Noncommunicable infections

Noncommunicable infections are those that are not transmitted from human to humans and include:

1. Infections derived from the patient’s normal flora or microflora such as peritonitis after rupture of the appendix;

2. Infections caused by the ingestion of preformed toxins, such as Botulism; and

3. Infections caused by certain microorganisms found in the environment, such as Clostridial gas gangrene.

Some zoonotic infections (diseases transmitted from animals to humans), such as Rabies and Brucellosis, are not transmitted between humans but others such as plague may be at certain stages.

Noncommunicable infections may still occur as common source outbreaks, such as food poisoning from an enterotoxin producing Staphylococcus aureus contaminated chicken salad or multiple cases of pneumonia from extensive dissemination of Legionella through an air conditioning system.

As these diseases are not transmissible to others, they do not lead to secondary spread.

Communicable infections

Communicable infections are transmissible from one person to the next person. They can be endemic, which simple means that the diseases are present at a low but fairly constant level, or epidemic, which now involves a level of infection above that usually found in a community or population.

Communicable infections may be widespread in a region and sometimes world wide with a high attack, in which case they are termed pandemic. A communicable infection requires that microorganisms to be able to leave the body in a form that either is directly infectious to others or is able to become so after development in a suitable environment.

An example of direct communicability is the respiratory spread of the Influenza viruses. In contrasts, the Malarial parasite requires a developmental cycle in a biting mosquito before another human being can be infected.

Infections and diseases

Infections involves multiplication of the microorganisms in or on the host and can be inapparent, for example, during the incubation period.Diseases are represented by a clinical apparent response by or injury to the host as a result of infection.

With many communicable microorganisms, infection is much more common than diseases, and apparently healthy infected individuals play an important role in disease propagation. Inapparent infections ate termed subclinical, and the individual is sometimes referred to as a carrier.

The latter term is also applied to situations in which an infectious agent establishes itself as part of a patient’s microflora or causes low grade chronic disease after acute infection. 

For example, the clinical inapparent presence of Staphylococcus aureus in the anterior nares is termed carriage, as is a chronic gallbladder infection with Salmonella typhoid that can follow an attack of typhoid fever and result in fecal excretion of the microorganisms for years.

With some infectious diseases, such as measles, infection invariably accompanied by clinical manifestations of the disease itself. These manifestations facilitate epidemiologic control, because the existence and extent of infection in a community are readily apparent.

Microorganisms associated with long incubation periods or high frequencies of subclinical infections like HIV or Hepatitis B viruses may propagate and spread in a population for long periods before the extent of the problem is recognized. This makes communicability control more difficult.

The incubation period and communicability

The incubation period is the time between exposure to the microorganisms and appearance of the first symptoms of the diseases.

Generally, microorganisms that multiply rapidly and produce local infections, such as Gonorrhea and influenza , are associated with short incubation periods (eg.2-4 days).

Diseases such as typhoid fever, which depends on hematogenous spread and multiplication of the microorganisms in distant target organs to produce symptoms, often have longer incubation periods.

Some diseases have even more prolonged incubation periods because of slow passage of the infecting microorganisms to the target organ, as in rabies or slow growth of microorganisms such as that of Tuberculosis.

Incubation periods for one agent may also vary widely depending on route of acquisition and infecting dose. For example, the incubation period of hepatitis B virus infection may vary from 7 to more than 200 days.

Communicability of a disease in which the microorganism is shed in secretions may occur primarily during the incubation period. In other infections the disease course is short but the microorganisms can be excreted from the host for extended periods of time.

In yet other cases, the symptoms are related to the host immune response tether than the microorganisms action, and thus the diseas process may extend far beyond the period in which the etiologic agent can be isolated or spread.

Some viruses can integrate into the host genomes or survive by replicating very slowly in the presence of an immune response.

Such dormancy or latency is exemplified by the Herpesviruses, and in each case the microorganisms may emerge long after the original infection and potentially infect others.


The inherent infectivity and virulence of microorganisms are also important determinants of attack rates of diseases in a community.

In general, microorganisms of high infectivity spread more easily and those of greater virulence are more likely to cause diseases than subclinical infection.

The infecting dose of the microorganisms also varies with different microorganisms and thus influences the chance of infection and development of a disease

Author: Elliot Zweli Jiyane

I am Elliot Zweli Jiyane, born in South Africa, Newcastle. All my education has been on medical issues. I'm a medical expert, holding multiple degrees under health care sector. From dentistry- medicine. I have published articles in some journal, my work is available on google scholar. I have work in several medical institutions. Feel free to communicate with me at any level of medical questions.

4 thoughts on “Sources and communicability of infectious diseases”

  1. While I can’t claim to be a medical professional, the information you’ve presented certainly seems to ring true to me. This is why I tend to wash my hands more than others do. XD

  2. This is great information, but definitely not for a hypochondriac. Although I took several health classes in College, it is nice to have a condensed resource with explanations. I have a few family members that can’t handle health conversations, so it is nice to have a source to keep my information accurate and up to date.

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