Agents which cause infectious disease

Instruct students to use the information they have learned from the infographic to describe what host their pathogen needs, how their pathogen would infect that host, the symptoms the pathogen would induce in the host, and explain how the infection could be prevented or treated.

Tell students to reference the infographic to make sure that their fictional pathogen aligns with the information cited. Encourage students to make a model or a drawing to accompany their written descriptions of their fictional pathogen.

Finish by having students present their work to the class. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society. National Geographic Society. For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher.

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You cannot download interactives. Droplet nuclei are dried residue of less than 5 microns in size. In contrast to droplets that fall to the ground within a few feet, droplet nuclei may remain suspended in the air for long periods of time and may be blown over great distances.

Vehicles that may indirectly transmit an infectious agent include food, water, biologic products blood , and fomites inanimate objects such as handkerchiefs, bedding, or surgical scalpels.

A vehicle may passively carry a pathogen — as food or water may carry hepatitis A virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin — as improperly canned foods provide an environment that supports production of botulinum toxin by Clostridium botulinum.

Vectors such as mosquitoes, fleas, and ticks may carry an infectious agent through purely mechanical means or may support growth or changes in the agent. Examples of mechanical transmission are flies carrying Shigella on their appendages and fleas carrying Yersinia pestis , the causative agent of plague, in their gut.

In contrast, in biologic transmission, the causative agent of malaria or guinea worm disease undergoes maturation in an intermediate host before it can be transmitted to humans Figure 1.

The portal of entry refers to the manner in which a pathogen enters a susceptible host. The portal of entry must provide access to tissues in which the pathogen can multiply or a toxin can act. Often, infectious agents use the same portal to enter a new host that they used to exit the source host.

For example, influenza virus exits the respiratory tract of the source host and enters the respiratory tract of the new host. Other portals of entry include the skin hookworm , mucous membranes syphilis , and blood hepatitis B, human immunodeficiency virus. Figure 1. The final link in the chain of infection is a susceptible host. For example, persons with sickle cell trait seem to be at least partially protected from a particular type of malaria.

Specific immunity refers to protective antibodies that are directed against a specific agent. Such antibodies may develop in response to infection, vaccine, or toxoid toxin that has been deactivated but retains its capacity to stimulate production of toxin antibodies or may be acquired by transplacental transfer from mother to fetus or by injection of antitoxin or immune globulin.

Nonspecific factors that defend against infection include the skin, mucous membranes, gastric acidity, cilia in the respiratory tract, the cough reflex, and nonspecific immune response. Factors that may increase susceptibility to infection by disrupting host defenses include malnutrition, alcoholism, and disease or therapy that impairs the nonspecific immune response.

Knowledge of the portals of exit and entry and modes of transmission provides a basis for determining appropriate control measures. In general, control measures are usually directed against the segment in the infection chain that is most susceptible to intervention, unless practical issues dictate otherwise.

For some diseases, the most appropriate intervention may be directed at controlling or eliminating the agent at its source. A patient sick with a communicable disease may be treated with antibiotics to eliminate the infection. An asymptomatic but infected person may be treated both to clear the infection and to reduce the risk of transmission to others.

In the community, soil may be decontaminated or covered to prevent escape of the agent. Some interventions are directed at the mode of transmission. Interruption of direct transmission may be accomplished by isolation of someone with infection, or counseling persons to avoid the specific type of contact associated with transmission.

Vehicleborne transmission may be interrupted by elimination or decontamination of the vehicle. To prevent fecal-oral transmission, efforts often focus on rearranging the environment to reduce the risk of contamination in the future and on changing behaviors, such as promoting handwashing. For airborne diseases, strategies may be directed at modifying ventilation or air pressure, and filtering or treating the air. To interrupt vectorborne transmission, measures may be directed toward controlling the vector population, such as spraying to reduce the mosquito population.

Some strategies that protect portals of entry are simple and effective. For example, bed nets are used to protect sleeping persons from being bitten by mosquitoes that may transmit malaria. Wearing of long pants and sleeves and use of insect repellent are recommended to reduce the risk of Lyme disease and West Nile virus infection, which are transmitted by the bite of ticks and mosquitoes, respectively.

Vaccinations promote development of specific antibodies that protect against infection. On the other hand, prophylactic use of antimalarial drugs, recommended for visitors to malaria-endemic areas, does not prevent exposure through mosquito bites, but does prevent infection from taking root.

Finally, some interventions attempt to prevent a pathogen from encountering a susceptible host. The degree of herd immunity necessary to prevent or interrupt an outbreak varies by disease. In theory, herd immunity means that not everyone in a community needs to be resistant immune to prevent disease spread and occurrence of an outbreak. One problem is that, in highly immunized populations, the relatively few susceptible persons are often clustered in subgroups defined by socioeconomic or cultural factors.

If the pathogen is introduced into one of these subgroups, an outbreak may occur. Information about dengue fever is provided on the following pages. After studying this information, outline the chain of infection by identifying the reservoir s , portal s of exit, mode s of transmission, portal s of entry, and factors in host susceptibility. Check your answer. Dengue is an acute infectious disease that comes in two forms: dengue and dengue hemorrhagic fever.

The principal symptoms of dengue are high fever, severe headache, backache, joint pains, nausea and vomiting, eye pain, and rash. Generally, younger children have a milder illness than older children and adults. Dengue hemorrhagic fever is a more severe form of dengue. It is characterized by a fever that lasts from 2 to 7 days, with general signs and symptoms that could occur with many other illnesses e. This stage is followed by hemorrhagic manifestations, tendency to bruise easily or other types of skin hemorrhages, bleeding nose or gums, and possibly internal bleeding.

This may lead to failure of the circulatory system and shock, followed by death, if circulatory failure is not corrected. Diagnosis of dengue infection requires laboratory confirmation, either by isolating the virus from serum within 5 days after onset of symptoms, or by detecting convalescent-phase specific antibodies obtained at least 6 days after onset of symptoms. There is no specific medication for treatment of a dengue infection.

Persons who think they have dengue should use analgesics pain relievers with acetaminophen and avoid those containing aspirin.

They should also rest, drink plenty of fluids, and consult a physician. Mosquitoes can carry the malaria parasite or West Nile virus. Deer ticks may carry the bacterium that causes Lyme disease. Disease-causing germs can also infect you through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. Escherichia coli E. While anyone can catch infectious diseases, you may be more likely to get sick if your immune system isn't working properly.

This may occur if:. In addition, certain other medical conditions may predispose you to infection, including implanted medical devices, malnutrition and extremes of age, among others.

Most infectious diseases have only minor complications. But some infections — such as pneumonia, AIDS and meningitis — can become life-threatening. A few types of infections have been linked to a long-term increased risk of cancer:.

In addition, some infectious diseases may become silent, only to appear again in the future — sometimes even decades later. For example, someone who's had chickenpox may develop shingles much later in life. Prepare food safely. Keep counters and other kitchen surfaces clean when preparing meals. Cook foods to the proper temperature, using a food thermometer to check for doneness. For ground meats, that means at least F 71 C ; for poultry, F 74 C ; and for most other meats, at least F 63 C.

Also promptly refrigerate leftovers — don't let cooked foods remain at room temperature for long periods of time.

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This content does not have an Arabic version. Overview Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Request an Appointment at Mayo Clinic. What are superbugs and how can I protect myself from infection? Share on: Facebook Twitter. Show references Facts about infectious disease. Infectious Disease Society of America.