Infectious Diseases (General)

By definition, those diseases that are transmitted from one person to another or between animals and humans (animal to animal, animal to human, human to human, human to animal), implying the transfer of a microorganisms or/and an infectious pathogen agent, are infectious diseases. Infectious agents are living (or at least nucleic acid encoding or proteins) units that must invade the insect host in order to initiate an infection.

Infectious diseases could be caused by bacteria, fungi, viruses, parasites (protozoan and helminthes) and prions. Ectoparasites produce infestation, a different term to express the presence of a pathogen agent on the external region of the body of the host.

Pathogens can be spread by many routes other than direct contact, including through water, food, air, and bodily fluids—blood, semen, saliva, and so on. Many diseases may be transferred by vectors—animals (usually insects) that carry microorganisms (mostly viruses and parasites). Vectors may spread a disease either by mechanical or by biological means (the organisms require a vector to reach specific stages of its life cycle, particularly the infectious stage for the intermediary, accidental and definitive hosts). Mechanical transmission occurs, when flies transfer the germs for typhoid fever from the feces (stool) of infected people to food eaten by healthy people. Biological transmission takes place when an insect bites a person and takes infected blood into its own system. Once inside the insect's gut, the dis-ease-causing organisms may reproduce, increasing the number of microorganisms that can be transmitted to the next victim. This is how the Anopheles spp. or Aedes spp. mosquito vector, for instance, transfers malaria or dengue, respectively. Additionally, the advance in new areas of activity has let the emergence of new infectious diseases as well new pathogens behavior, and the modification of established methods of transmission. In the medical management of patients, particularly in hospitals, healthcare-associated infections are also a raising concern worldwide (nosocomial infections).

Historically, the record of human suffering and death caused by smallpox, cholera, typhus, dysentery, malaria, and so forth establishes the eminence of the infectious diseases. Despite the outstanding successes in control afforded by improved sanitation, immunization, and antimicrobial therapy, the infectious diseases continue to be a common and significant problem of modern medicine. The most common disease of humankind, the common cold, is an infectious disease, as is the feared modern disease AIDS. Some chronic neurological diseases that were thought formerly to be degenerative diseases have proven to be infectious. Day by day, news scientific findings have revealed the role of the infectious agents in chronic diseases such as asthma, multiple sclerosis, some chronic cardiovascular diseases, Whipple disease, gastric diseases, diabetes, and many types of cancer, among others. There is little [Page 910]doubt that the future will continue to reveal the infectious diseases as major medical problems.

The infectious agent is carried by healthy hosts called reservoirs (e.g., water birds for the flu virus). It is transmitted to the target organism either directly or through carrier organisms. For example, the protozoa responsible for malaria live in mosquitoes, carriers that transmit them through their bites to humans, who develop the disease. The worldwide emergence of diseases once restricted to a particular region is caused by changes in lifestyle and human modification of the environment due to high demographic growth (clearing and deforestation, contact with wild fauna, overcrowding in megalopolises, tourism, immigration, etc.).

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