Most people on Earth experience at least one episode of an infectious disease every year. Although the majority recover, hundreds of millions suffer severe or long-term health effects as a direct result of an infection and around 10 million people – many of them children – lose their lives. In the 1960s, it was widely believed that advances in methods of prevention and treatment would overcome the threat to health from infectious diseases. Unfortunately these predictions have proved to be optimistic because of the rapidly increasing threat from ‘emerging infectious diseases’.
- 7.2: What are infectious diseases?
- Infectious diseases are distinguished from other illnesses and disorders because they can be transmitted from someone who is ill either directly or indirectly to other individuals, who then develop the same infectious disease and are also able to pass it on. A familiar example is the ‘common cold’ which almost everyone has experienced at some time in their lives . Non-human animals and plants also suffer from infectious diseases, which cause massive losses to food crops and livestock.
- 7.4: What causes infectious diseases?
- Infectious diseases are transmitted between individuals by infectious agents, known as pathogens, from the Greek word pathos (to suffer) and genès (to produce). Pathogens produce a lot of human suffering and disability across the world, including in relatively wealthy nations like the United States. Most people have heard of at least some types of pathogen, for example bacteria or viruses.
- 7.5: Direct person-to-person transmission of pathogens
- A new infection begins when pathogens leave the body of their host – the infected individual in which the pathogens are multiplying – and enter a new host. They may be repelled by defense mechanisms (i.e. skin) in the new host, or they may survive and reproduce in sufficient numbers to cause an infectious disease.
- 7.6: Indirect person-to-person transmission of pathogens
- There are many routes for indirect person-to-person transmission of infection. Most airborne infections are transmitted when a cough or sneeze expels fine droplets of water (known as an aerosol) containing millions of bacteria or viruses. The aerosol droplets may be inhaled by a susceptible person, or settle on surfaces where the pathogens contaminate hands, utensils, clothing, water or food, which are then touched or consumed by someone else.
- 7.7: Animal-to-human transmission of pathogens
- Pathogens are often transmitted from animals to humans ‘accidentally’, for example via infected meat or water contaminated with animal feces. But there are two transmission routes in which the animal is an essential agent in the transfer of pathogens to humans. An example of zoonosis that may be familiar is influenza originating in pigs (swine flu) or poultry (bird flu).
Thumbnail: Colored Condoms. Condom use is one of the best ways to promote and practice safe sex. Image used with permission (Public Domain; National Institutes of Health, part of the United States Department of Health and Human Services.)