Definition of Infectious Diseases

Infectious disease is disease caused by a pathogen which enters the body and triggers the development of an infection. Infectious diseases have a range of causes, and they can be found all over the world. These diseases are considered contagious or communicable, meaning that they can be passed from person to person. It is also possible for such diseases to spread indirectly through unhygienic conditions, or from animals to people, in which case they are known as zoonotic diseases.

A variety of pathogens can be responsible for infectious disease, including viruses, bacteria, fungi, protozoans, and prions. Within these large categories of infectious organisms, there are numerous modes of transmission and a colorful assortment of symptoms, although surprisingly few organisms cause infectious disease, when one considers the diversity of viral, bacterial, fungal, and protozoan life. In order to treat an infectious disease, doctors must be able to knock out the source of the infection and repair the damage it has done to the body. Many infectious diseases make the body vulnerable to secondary infections, in which other organisms move in to take advantage of a weakened immune system, and this can be very problematic.

The study of infectious disease is known as epidemiology. Epidemiologists work to determine the source of an infectious disease so that they can develop new treatment approaches. They also identify emerging outbreaks, which may develop into epidemics or pandemics, and areas where a disease is endemic, meaning that it occurs regularly. Malaria, for example, is endemic to some regions of Africa and Southeast Asia. Read more »

Mycoplasma Infections

Background

Mycoplasma species are the smallest free-living organisms. These organisms are unique among prokaryotes in that they lack a cell wall, a feature largely responsible for their biologic properties such as their lack of a reaction to Gram stain and their lack of susceptibility to many commonly prescribed antimicrobial agents, including beta-lactams. Mycoplasmal organisms are usually associated with mucosal surfaces, residing extracellularly in the respiratory and urogenital tracts. They rarely penetrate the submucosa, except in the case of immunosuppression or instrumentation, when they may invade the bloodstream and disseminate to different organs and tissues throughout the body.

Although scientists have isolated at least 17 species of Mycoplasma from humans, 4 types of organisms are responsible for most clinically significant infections that may come to the attention of practicing physicians. These species are Mycoplasma pneumoniae, Mycoplasma hominis, Mycoplasma genitalium, and Ureaplasma species. The focus of this article is infections caused by M pneumoniae; articles on Ureaplasma infections (eg, Ureaplasma Infection) and genital mycoplasmal infections contain discussions of infections caused by other mycoplasmal species.

Pathophysiology

M pneumoniae is perhaps best known as the cause of walking or atypical pneumonia, but the most frequent clinical syndrome caused by this organism is actually tracheobronchitis or bronchiolitis, often accompanied by upper respiratory tract manifestations. Pneumonia develops in only 5%-10% of persons who are infected. Acute pharyngitis and myringitis are less common.

After inhalation of respiratory aerosols, the organism attaches to host cells in the respiratory tract. The P1 adhesin and other accessory proteins mediate attachment, followed by induction of ciliostasis, local inflammation that consists primarily of perivascular and peribronchial infiltration of mononuclear leukocytes, and tissue destruction that may be mediated by liberation of peroxides. Recently, M pneumoniae has been shown to produce an exotoxin that is believed to play a role in the damage to the respiratory epithelium that occurs during acute infection. The organism also has the ability to exist intracellularly. Additionally, acute mycoplasmal respiratory tract infection may be associated with exacerbations of chronic bronchitis and asthma. More extensive information on the pathogenesis of mycoplasmal respiratory infections is available in a recent review article.

Spread of infection throughout households is common, although person-to-person transmission is slower than for many other common bacterial respiratory tract infections; close contact appears necessary. Generally, the incubation period is 2-3 weeks. The organism may persist in the respiratory tract for several months, and sometimes for years in patients who are immunosuppressed, after initial infection. Read more »