Communicable diseases: what they are and how they are classified

First epidemic manifestations of the COVID-19 pandemic in Italy date back to January 30th 2020, when two tourists from China tested positive for the SARS-CoV-2 virus in Rome.

Covid-19 pandemic
Figure 1 – Covid-19 pandemic [credits:]

Subsequently, an outbreak of infections was detected from 16 confirmed cases in Lombardy on 21 February, which increased to 60 the following day with the first deaths reported on the same days.

But why is there talk of a pandemic? What are the classification criteria for communicable diseases? And above all, what are they?

Communicable diseases

Communicable disease” means a disease whose causative agent can be transferred from one individual to another.

Communicable diseases
Figure 2- communicable diseases [credits:]

First of all, a distinction must be made between the terms source, reservoir, infection vector and vehicle:

  • The source of disease is defined as the person, man or animal, who eliminates parasites allowing them to be transmitted to others by air, fecal-oral route, etc;
  • On the other hand, a reservoir is the subject that houses the etiological agent which cannot be transmitted unless there is the intervention of vectors or external agents (eg malaria). The definition of environmental reservoir which constitutes the animal, plant or inanimate substrate in which etiological agents can survive and multiply should also be clarified;
  • vehicle, or mechanical vector, is the medium, which can be an organism or an object, which passively carries pathogens (eg pen cap);
  • An infection vector, also called a biological vector or host, is an animated medium (living organism) that allows the transmission of a pathogen from one individual to another. The host is essential for transmission because part of the parasite’s life cycle takes place in the vector itself

Sick and carrier

A man or animal can be sick or a carrier of the disease.

In particular, in case that he is ill, he manifests the signs and symptoms of the pathology. Specifically, regarding the carriers we distinguish: early, healthy, chronic and convalescent carriers:

  • Early carriers can be a source of infection without knowing it because they do not yet show the signs and symptoms of the disease (this is the case of many sexually transmitted diseases such as gonorrhea whose signs and symptoms appear about a week after the germ lodges in the organs sexual).
  • On the other hand, healthy carriers is the case, for example, of individuals suffering from Salmonellosis which does not carry obvious signs or symptoms, but the subject emits germs into the environment and can transmit them;
  • Chronic carriers in which the disease does not completely heal, but tends to become chronic; they can, therefore, transmit the disease, as is the case, for example, of hepatitis C;
  • Finally, Convalescent carriers are those who are healed but continue to emit germs. This is the case, for example, with measles.

General classifications

Communicable diseases are classified according to their likelihood of occurring and the ways in which they manifest. The various trends are related to the nature of the etiological agent (eg. Difficult to transmit or easy to transmit).

Sporadic, endemic, epidemic, pandemic diseases

  • Sporadic diseases: they manifest themselves as isolated cases at a distance of time and space. They are supported by germs that have difficulty in transmitting. An example is determined by the rabies transmitted through the bite of the dog which does not show the obvious signs of the disease as it is an early carrier. In this case the germ dwells only in its jaws and can, therefore, be transmitted to a few people;
  • Endemics: they spread on average and are dictated by germs that have a discrete contagion capacity, but, for various reasons (such as the presence of vaccinated people, the resistance of the population, etc.) they spread in a more subtle way with a certain number of cases in the arc of the year. These diseases are always present in the area (eg measles);
  • Epidemics: starting from a certain number of cases, there is a notable expansion of the latter (eg typhus). Diseases with a certain ease of transmission are epidemic. These diseases, based on the nature of the causative agent, are divided into endemic, when the germ is normally present in the area, and exotic, when the germ is not normally present in the area (eg cholera in Italy). If the disease is exotic, a few cases are enough to raise the alarm, as this finds an unprotected population;
  • Pandemic: It is an epidemic that spreads over different areas. Highly contagious germs support a pandemic, the most classic example being that of the flu virus.

Original article “Malattie trasmissibili: cosa sono e come si classificano?

written and translated by Giovanna Spinosa

Images’ credits
  • Figure 1:;
  • Image 2:
Foto dell'autore

Giovanna Spinosa

Da sempre amante della microbiologia, sono laureata in scienze biologiche all'Università Federico II di Napoli ed attualmente studio diagnostica molecolare presso la medesima istituzione. Sono un membro del team BM Innovations dove, insieme ai miei colleghi, mi occupo di ricercare e sviluppare metodi innovativi per la salvaguardia dell'ambiente e non solo, sfruttando la bioinformatica.

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