Infectious Disease Area

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Infectious disease FAQs: learn more about this topic

Infectious diseases are communicable disorders caused by the spread of pathogenic organisms — such as bacteria, viruses, fungi or parasites — or proteins — such as prions — from human-to-human, animal-to-human or from the environment, including food and water. Examples include cholera, which is transmitted by contaminated food or water, and Ebola, which can spread through contact with bodily fluids.

Different infectious diseases have different routes of transmission, cause different symptoms and can be treated and/or prevented by different interventions. Take a look below as we review some of the most frequently asked questions on this topic.

What is the World Health Organization definition of infectious disease?

According to the World Health Organization: “Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi”. Many microorganisms live in or on our body normally, and are completely harmless. However, infectious diseases occur when some organisms cause disfunction under certain conditions.

How are infectious diseases spread?

Infectious diseases can be spread by a variety of methods. Many infectious diseases are spread either directly or indirectly from person to person. Direct transmission occurs when an infected person touches or exchanges bodily fluids with an uninfected person. For example, in sexually transmitted diseases, such as HIV and chlamydia, sexual contact is responsible for transmission. Examples of indirect contact includes contaminated food or water, as seen in E. coli transmission, contaminated objects or insect bites.

Infectious diseases can also be transmitted from animals to humans, and these disease are often referred to as zoonoses.

Are infectious diseases on the rise?

The World Health Organization reports that infectious diseases are responsible for over 17 million deaths every year. Changing socioeconomic and environmental factors (in addition to news drugs and vaccines) have led to a rise in some infectious diseases, and a decrease in others.

For example, the Centers for Disease Control have demonstrated a three-fold increase in the number of diseases transmitted by mosquitoes, ticks and fleas between 2004 and 2016. These disease include West Nile, Zika, malaria and Lyme disease. Climate change has led to a concern that these diseases could spread further as the vectors that carry them are increasingly able to populate new areas.

In addition, the ease of travel in the modern world adds another level of complexity for infectious disease; new cases could be a plane ride away. Finally, an aging population leads to not only an increase in chronic conditions but also an increased susceptibility to infections.

However, other diseases, such as smallpox and plague, that were once major killers no longer are.

How can infectious diseases be prevented?

Vaccines are available to prevent many common infectious diseases, for example, human papillomavirus, diphtheria, measles and polio. These are substances used to stimulate the immune system into producing antibodies, thus providing immunity against a disease.

In addition to vaccination, simple preventative measures such as frequent hand washing can reduce the risk of disease transmission.

Which infectious diseases have been eradicated?

The world has eradicated just two diseases: smallpox and rinderpest. Smallpox was eradicated in 1980 and rinderpest was declared eradicated in 2011.

Diseases that are considered eradicable have humans as the only (or major) host for the disease, have an effective vaccine or treatment available and have both financial and political support for the eradication efforts.

Examples of diseases that are currently considered eradicable include: polio, lymphatic filariasis, polio, mumps, measles, cysticercosis and rubella. Efforts to eradicate other infectious diseases such as malaria, rabies, trachoma and yaws are also underway.

Which infectious diseases are reportable?

Reportable, or notifiable, diseases are those that when detected should be immediately reported to the relevant public health body – for example, the Centers for Disease Control and Prevention in the USA or Public Health England in the UK. These diseases are considered to be of great public health importance. Reporting allows for any outbreaks to be monitored, acted on quickly and for trends to be identified. Information gathered from this reporting allows authorities to make informed decisions on infection control activities.

As an example, diseases reportable to the Centers for Disease Control and Prevention include anthrax, botulism, cholera, giardiasis, HIV infection, leprosy, Lyme disease, meningitis, polio, rabies, rubella, SARs, typhoid fever, yellow fever and Zika.

Which infectious diseases can be prevented by vaccinations?

Vaccine-preventable currently include diphtheria, tetanus, polio, pertussis, measles, mumps, rubella, hepatitis B, influenza, pneumococcal infections and Haemophilus influenzae type b infections. Vaccines have been developed for other diseases although these might only be given to sub-groups of a population or individuals in specific situations. Vaccines are under development for many other diseases.

Which infectious disease has the highest mortality rate?

Worldwide, tuberculosis is the leading cause of death from a single infectious agent. In 2018 it was estimated that 10 million people fell ill with tuberculosis and 1.5 million individuals died from the disease. However, the mortality rate for tuberculosis is falling at approximately 3% per year, and between 2000 and 2017 reduced by 42%.

Some infectious diseases have a higher mortality rate than this, for example, Creutzfeldt-Jakob disease – a neurodegenerative disease caused by prions – is 100% fatal. Moreover, rabies is also 100% fatal if it is not treated rapidly.

This image shows viruses, one of the causative agenst of infecitous disease