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 »

From the infectious diseases meeting: What’s with the vaccine-o-phobia?

PHILADELPHIA – For the folks who promote vaccination, these are trying times. Recently, CNN hosted a segment titled: “Virus or Vaccine: Which is Worse?”

It’s enough to set Paul Offit to ranting, which he did this week at a meeting of the Infectious Diseases Society of America. Offit, a physician who heads the infectious disease division at Children’s Hospital of Philadelphia, has devoted a career to fighting illness. In his job, vaccines are often the most reliable weapon available, and cost-effective to boot. And although it’s astonishingly more dangerous to contract a disease than it is to get vaccinated for it, that message seems to have gotten lost somewhere along the way.

Offit traces this detour back to 1982, when DPT — the shot that prevents diphtheria, tetanus and pertussis – was (wrongly) linked to brain damage. “Three people believed their kids were harmed by the vaccine,” he says.

Offit has compassion for families who have a child who has suffered, whatever the cause may be, known or unknown. But since 1982, it’s been one accusation after another against vaccines. People tried to link the HIB vaccine to diabetes (no evidence), the hepatitis B vaccine to multiple sclerosis (all but one study found no link), and other vaccines to SIDS or autism. Recently, the HPV vaccine — which prevents cervical cancer – got linked to heart attacks and strokes (no proof).

And now the seasonal flu vaccine and H1N1 flu vaccine are being skipped by millions of people who somehow distrust the science that went into making them, even though the illnesses they cause can be fatal. 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 »

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 »