Understanding Communicable vs Non-Communicable Diseases

Every disease fits into one of two camps: communicable and non-communicable disease, with the term communicable meaning the disease can be spread from one person or animal to another. Communicable diseases include the common cold, influenza, HIV, malaria, tuberculosis, plague, to name a few. Non-communicable diseases include cancer, diabetes, asthma and cardiovascular conditions.

Some diseases spread directly from animals to humans and are known as zoonotic diseases, or zoonoses. The category typically only includes diseases spread from vertebrate animals: Ebola, for example, is introduced into human populations by contact with infected wild animals such as monkeys or fruit bats, and influenza viruses are known to cross over from animals, including birds.

All communicable diseases are infectious, but only diseases that spread by direct contact from person to person can be called contagious, a common mistake people make, using the two terms interchangeably. All communicable diseases are spread through a pathogen, a bacterium, virus, parasite or any other microorganism capable of spreading disease.

When working within public health to control either class of disease, teams need to understand the epidemiology of the disease at hand, which means understanding the patterns of a disease among populations to know how it affects different groups of people and why. Knowing this is the first step in planning how best to stop it.

A person infected with a disease is known as a case. When an outbreak occurs, it begins with the first case noticed by health officials, the person who first indicates the presence of an outbreak, known as the index case. This is not to be confused with the primary case, which is the first person to bring an infection into a group of people or community, but may not be the first person brought to the attention of health officials to signify an outbreak has begun.

Outbreak, officially defined as the ‘sudden appearance of a disease in a specific geographic area or population’ by the US Centers for Disease Control, others including WHO expand on this to define it as cases of a disease being ‘in excess of what would normally be expected in a defined community, geographical area or season’. For example, a town may see a few cases of measles every now and then, but a sudden group of cases being reported would be an outbreak. With rare diseases, just two linked cases could be sufficient to be called an outbreak.

The next step up from an outbreak would be an epidemic, which carries the same definition regarding there being more cases of a disease than would be expected, but typically applies across a larger geographical area and often involves the rapid spread of the disease or issue across a population. The millions of people living with HIV globally constitute an epidemic, for example, as do the millions that continue to become infected with malaria and tuberculosis each year. A sudden rise in cases of any of these diseases in a particular location over a period of time would also be called an epidemic.

Some countries and regions are also endemic for certain infectious diseases, meaning they have continuous transmission of the disease. Malaria is endemic in more than 80 countries around the world according to the WHO, mostly in Africa, Asia and the Americas, while the eye infection trachoma is endemic in more than 40 countries, causing blindness or visual impairment in almost 2 million people.

When endemic countries experience sudden outbreaks or epidemics, control efforts become more complicated, and political, with routine control measures needing to be supplemented with more rapid response strategies. Here, simply identifying an outbreak can be complex as it could be easily missed or disregarded as a new, or separate, concern.

At times, the term cluster may also be used, referring again to the number of cases of a disease being above what is ‘normally expected’ but with cases being aggregated, close together, in time and location. Once investigated, they may go on to be classed as outbreaks if considered to be a cause for concern.

Pandemic, defined by the US CDC as ‘an epidemic occurring over a very large geographic area’, the WHO adds the detail of it being the worldwide spread of a ‘new disease’. This latter point would explain why diseases with global epidemics, such as malaria, tuberculosis, and dengue fever, for example, are not considered pandemics.

SARS and HIN1 swine flu were pandemics, as was the 1918 Spanish flu, also an HINI influenza virus. Most recently, the WHO added COVID-19 to the list. The finer points of what constitutes a pandemic are, unsurprisingly, unclear and are debated by scientists and health agencies alike, with experts arguing that factors such as novelty, severity, how easily a disease spreads, and attack rates – the proportion of people who fall ill once exposed – should come into play.

The time between someone being exposed to an infection and becoming infected is known as the latent period. The bug has not yet caused symptoms and typically cannot yet be transmitted to others. The time between exposure and developing symptoms of the disease is the incubation period. These first two periods overlap and are important to understand when it comes to disease control as cases may arise in new places, among newly susceptible people, before they realize they are infected.

The period when an infected person can transmit the disease is known as the infectious or contagious period. This can be before or after symptoms begin, depending on the disease. Many diseases are transmitted only once symptoms begin, so the contagious period begins after the incubation period – examples include Ebola and tuberculosis.

Measles can be transmitted once first symptoms begin, such as a fever, but four days before a rash develops – and for four days after the rash has gone. Bacterial meningitis can spread seven days before symptoms begin, meaning the latent and incubation periods overlap. Understanding this for a target disease informs contact tracing in order to identify who else may be at risk.

Source – Outbreaks and Epidemics: Battling Infection in the Modern World by Meera Senthilingam, Brian Clegg (Series Editor)

Goodreads – https://www.goodreads.com/book/show/44784435-outbreaks-and-epidemics

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