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 »

Male circumcision reduces risk of HIV transmission from women

In the study, published in the Feb. 15 issue of The Journal of Infectious Diseases, now available online, Jared Baeten and colleagues from the United States and Kenya collected detailed sexual data from a group of male Kenyan truckers and, using statistical models, developed infectivity measures that estimate the per-sexual-act probability of HIV transmission. The study is the first to calculate the probability of infection for men who have multiple, concurrent heterosexual partners, which was found to be significantly higher than infectivity rates calculated in the past from studies of monogamous couples. Their results may help explain the rapid spread of HIV in settings where circumcision is not common and multiple sexual partnerships are.

Between 1993 and 1997, 745 male employees of trucking companies based in Mombasa, Kenya were followed for the study. Initially they were evaluated for circumcision status and HIV-negativity. Over the length of the study the men were asked to give information concerning the number of sexual encounters with three different partner types–wives, casual partners, and prostitutes–and were screened for HIV and other sexually transmitted infections. At the end of the study the probability of infection was calculated using a statistical model that incorporated published data to estimate the rates of HIV infection among the three types of sexual partners.

For the men in the study, the overall probably of becoming HIV-infected following a single act of intercourse was calculated to be .0063, or one in 160. Uncircumcised men had a more than two-fold increased risk of infection per sexual act compared with circumcised men–one in 80 versus one in 200. Read more »

Malaria

Background
Malaria, which predominantly occurs in tropical areas, is a potentially life-threatening disease caused by infection with Plasmodium protozoa transmitted by an infective female Anopheles mosquito vector. Individuals with malaria may present with fever and a wide range of symptoms.

The 4 Plasmodium species known to cause malaria include Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae. A fifth species, Plasmodium knowlesi, has recently been identified as a clinically significant pathogen in humans. Timely identification of the infecting species is extremely important, as P falciparum infection can be fatal and is often resistant to standard chloroquine treatment. In some cases, individuals with malaria are infected with multiple Plasmodium species. P falciparum and P vivax are responsible for most new infections. Each Plasmodium species has a defined area of endemicity, although geographic overlap is common. Species can usually be distinguished by morphology on a blood smear. P falciparum is distinguished from the rest of plasmodia by its high level of parasitemia and the banana shape of its gametocytes.

Malaria in travelers typically manifests weeks after the individual leaves the endemic area. Presentation more than 4 weeks after returning from the endemic area is unusual. In some individuals, disease manifests months or years later, usually due to the presence of P vivax or P ovale hypnozoites, which can remain dormant in the liver and reactivate years after infection. Relapse with P vivax or P ovale infection is rare more than 5 years after initial infection. Because symptomatic delay is common, history of even a remote exposure to an endemic area should be elicited. Symptoms of malaria are nonspecific, and, because timely diagnosis and treatment are necessary, malaria should be considered in all patients from tropical areas who present with fever. Read more »

Dengue Fever

Background
Dengue, the most common arboviral illness transmitted worldwide, is caused by infection with 1 of the 4 serotypes of dengue virus, family Flaviviridae, genus Flavivirus (single-stranded nonsegmented RNA viruses). Dengue is transmitted by mosquitoes of the genus Aedes, which are widely distributed in subtropical and tropical areas of the world, and is classified as a major global health threat by the World Health Organization (WHO).

Initial dengue infection may be asymptomatic (50%-90%), may result in a nonspecific febrile illness, or may produce the symptom complex of classic dengue fever (DF). A small percentage of persons who have previously been infected by one dengue serotype develop bleeding and endothelial leak upon infection with another dengue serotype. This syndrome is termed dengue hemorrhagic fever (DHF), although dengue vasculopathy has been proposed as a better term, as fluid loss into tissue spaces can lead to prolonged shock and complications, including gastrointestinal bleeding, a greater fatality risk than bleeding per se. Some patients with dengue hemorrhagic fever develop shock (dengue shock syndrome [DSS]), which may cause death.

Dengue virus transmission follows two general patterns—epidemic dengue and hyperendemic dengue. Epidemic dengue transmission occurs when dengue virus is introduced into a region as an isolated event that involves a single viral strain. If the number of vectors and susceptible pediatric and adult hosts is sufficient, explosive transmission can occur, with an infection incidence of 25%-50%. Mosquito-control efforts, changes in weather, and herd immunity contribute to the control of these epidemics. Transmission appears to begin in urban centers and then spreads to the rest of a country. This is the current pattern of transmission in parts of Africa and South America, areas of Asia where the virus has reemerged, and small island nations. Travelers to these areas are at increased risk of acquiring dengue during these periods of epidemic transmission.

Hyperendemic dengue transmission is characterized by the continuous circulation of multiple viral serotypes in an area where a large pool of susceptible hosts and a competent vector (with or without seasonal variation) are constantly present. This is the predominant pattern of global transmission. In these populations, antibody prevalence increases with age and most adults are immune. Hyperendemic transmission appears to be a major risk for dengue hemorrhagic fever. Travelers to these areas are more likely to be infected than are travelers to areas that experience only epidemic transmission. Read more »

Tetanus

Background

The word tetanus comes from the Greek tetanos, which is derived from the term teinein, meaning to stretch. Tetanus appears in military medical documents throughout the ages. Slapping infected dung on the umbilical cords of newborns (ie, as part of ritualistic ceremonies) caused rampant tetanus neonatorum or trismus nascentium in the West Indies and in Africa. Osler’s textbook describes the “eight days sickness” caused by umbilical sepsis, which killed 84 of 125 children within a fortnight of birth in St. Kilda, Scotland. During World War I, tetanus occurred in 1.47 per 1000 British wounded and in 12.5 per 1000 persons involved in the Peninsular campaign. Nicolaier discovered the anaerobic bacillus Clostridium tetani in 1885. In 1889, Koch’s pupil, Kitasato, obtained the bacillus of tetanus in pure culture and associated the disease to animals.

Although rare, the disease has not been eradicated, and early diagnosis and intervention are life saving. Prevention is the ultimate management strategy for tetanus. The 4 clinical types of tetanus are generalized, local, cephalic, and neonatal.

Neonatal tetanus is a major cause of infant mortality in underdeveloped countries, but this form is rare in the United States. Infection results from cord contamination during unsanitary delivery conditions, coupled with a lack of maternal immunization. At the end of the first week of life, infected infants become irritable, feed poorly, and develop rigidity with spasms. This form of tetanus has a very poor prognosis for survival.

Cephalic tetanus is uncommon and usually occurs following head trauma or otitis media. Patients with this form present with cranial nerve palsies. The infection may be localized or may become generalized.

Patients with local tetanus present with persistent rigidity in the muscle group close to the injury site. The muscular rigidity is caused by a dysfunction in the interneurons that inhibit the alpha motor neurons of the affected muscles. No further CNS involvement occurs, and this form has very low mortality rates. Read more »