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aAll are bacteria, except Plasmodium, a protozoan.

A disease mainly of animals that also can be transmitted to humans (such as anthrax) is called a zoonosis (plural, zoonoses). Some infectious agents are able to survive in the environment, outside any host. This includes some species, referred to as opportunistic pathogens, which normally grow in soil or water or other environments but are capable of infecting a host that is compromised (weakened) by an injury, condition, or disease.

The human reservoir may include both people with the disease and those who are infected but show no symptoms. Individuals with such subclinical infections are referred to as asymptomatic carriers. The most notorious example of a carrier was Mary Mallon, known as "Typhoid Mary.'' Although she showed no symptoms herself, she was the source of a number of typhoid outbreaks in the New York City area in the early twentieth century. Because she refused to stop working as a food handler, eventually she was jailed.

One source of infection is the native microorganisms living on or in an individual. As a result of some change in it (increased virulence, the ability to cause disease) or (more commonly) the host, this usually innocuous indigenous microbe may become pathogenic. For example, the common intestinal Escherichia coli can acquire a virulence factor leading to severe diarrhea, the common skin inhabitant Staphylococcus aureus can cause serious illness after being transferred into formerly sterile tissue as the result of a subcutaneous wound penetration, or a fungal vaginal infection can result from suppression of the normally dominant bacterial populations because of antibiotic use.

However, in most cases an infectious agent must be transmitted from one host to another. This transmission may be relatively direct or may involve an intermediate. Direct transmission occurs through contact or exchange of bodily fluids between an infected host and the new, previously noninfected host. Rabies and sexually transmitted diseases such as syphilis are spread in this way. Many skin diseases, such as ringworm, are also transmitted via direct contact, but they may also be spread indirectly on objects (such as towels) because the causative agent can persist in the environment for a sufficient period of time. Most respiratory diseases are droplet infections, spread through aerosols (liquid or solid particles in air) resulting from exhaling, sneezing, or coughing. These remain suspended briefly in the air (during which time the agents remain viable), then are inhaled by the new host. Thus, the transmission is not truly direct, but is still generally considered to be so because the time in the air is so short (minutes or less). It is now believed that the common cold often is spread by direct contact through the following scenario: an infected person's hand becomes infectious when it touches the fluids of his or her mouth, nose, or eye; transmission occurs when he or she touches another person's hand; the new host becomes infected when touching his or her own mouth, nose, or eye.

Indirect transmission occurs through some other medium. A nonliving material that is capable of infecting a large number of individuals is referred to as a vehicle. Food and water are the two most common. Air can also be considered a vehicle, provided that the infectious agent is able to survive in this usually hostile (mainly because of drying and ultraviolet radiation) environment. Other nonliving means of transmission, such as clothing, furniture, toys, doorknobs, and bandages, are referred to as fomites.

A living intermediate for indirect transmission is called a vector. Many common vectors are biting insects (e.g., mosquitoes, malaria and some types of encephalitis; fleas, plague; lice, typhus; and flies, sleeping sickness) or ticks (Lyme disease and Rocky Mountain spotted fever). The vector picks up the infectious agent when it bites an infected host, then transmits it to a new host with another bite. In many cases the infectious agent reproduces within the vector (which is then considered an alternate host), thus increasing the likelihood of successful transmission to the next host. However, in some cases an organism such as a nonbiting fly is simply a mechanical vector, transporting the infectious agent from host to host on its mouth parts, feet, wings, or body hairs.

Not every transmission of a pathogen to a new potential host results in infection and disease. First, the host must be susceptible—a species that the pathogen can parasitize, and without previously developed immunity (resistance) from vaccination or prior exposure. Also, at least a minimum quantity of pathogens, called an infective dose, must be transmitted. Although for a few pathogens (perhaps the virus hepatitis A or the round-worm Ascaris) only a single viable particle (e.g., cell, spore, cyst, egg) may be sufficient, it more commonly takes at least tens (e.g., the bacterium Shigella and the protozoans Entamoeba histolytica, Cryptosporidium parvum, and Giardia lamblia), thousands (Vibrio cholerae), or even millions (e.g., Salmonella, Clostridium perfringens) of pathogens to overcome a healthy body's defense mechanisms and produce disease.

Example 12.1 While hiking in a national forest, a group of healthy young adults drinks from a clear mountain brook. Unknown to them, a short distance upstream there is a colony of beaver that has been infected with Giardia lamblia, and the water they consumed contains 2 viable cysts/mL. How much could a person probably drink without developing giardiasis, assuming that the infective dose is 20 cysts?

Solution If the number of cysts likely to result in disease is 20, a person could drink

20 cysts ^ 2 cysts/mL = 10 mL or about 2 teaspoons. Thus the hikers are likely to develop giardiasis.

Not all people show equal resistance to disease, and individual resistance also varies over time. In general, the very young and very old have weaker immune systems, and some diseases may also severely compromise the ability to fight off other infections. This is of special concern in hospitals, since patients are typically in poorer health than is the general population, and thus more susceptible to contracting additional diseases. A nosocomial infection is one that is acquired in a hospital. In the United States alone, about 100,000 deaths per year occur from such infections, making them the fourth highest killer here (after heart disease, cancer, and strokes).

Parasites and their hosts evolve together over time. In general, it is not in the "interest" of a pathogen to kill its host. In fact, some fatal diseases, such as rabies and plague, are actually zoonoses, with humans an accidental (and therefore unadapted) host. Cholera, which is a human disease, appears to have grown somewhat milder over the last two centuries. Other human diseases remain fatal, however, perhaps because they have not sufficiently evolved yet. Also, previously unexposed populations may be particularly susceptible to a disease. This was the case, for example, among the indigenous peoples of North America when smallpox and measles were introduced from Europe.

Routes of Infection There are several logical ways in which to classify diseases. One way is by the region of the body that is infected, such as respiratory disease, gastrointestinal illness, or urinary tract infection. Another is to group diseases by their causative agents, such as diseases caused by particular types of bacteria or viruses. However, for environmental engineers and scientists, it is usually most helpful to categorize diseases by their mode of transmission.

That is the approach that is taken here. In the following sections we look at diseases transmitted by water, food, air, vectors, sexual activity, and other direct contact. This is somewhat arbitrary, since a single disease may be spread by more than one route (e.g., water and food). The greatest emphasis is placed on waterborne disease, since this is where the role of the environmental engineer and scientist is greatest.

Table 12.3 provides an alphabetical listing of many diseases, along with their causative agents and mode of transmission. Although the list is certainly not all-inclusive, an effort was made to list all diseases mentioned in this and other chapters.

TABLE 12.3 Selected Diseases of Humans, Their Causative Agents, and Major Modes of Transmission

Disease Microbial Agent Taxaa Transmissionb Comments

Abscesses, boils Staphylococcus b-7 C-w; I Most common agent aureus

African sleeping sickness

Trypanosoma gambiense, T. rhodesiense p-f

V-tsetse fly Trypanosomiasis

Amoebic dysentery

Entamoeba histolytica p-a

Amebiasis

Anthrax

Bacillus anthracis b-7

Zoonosis—sheep, cattle; usually contracted through cuts in skin; airborne as biological weapon

TABLE 12.3 (Continued)

Disease

Microbial Agent Taxaa

Transmissionb

Comments

Aquired immune deficiency syndrome (AIDS) Ascariasis

Aspergillosis

Athlete's foot, jock itch Bacterial dysentery Botulism

Bovine spongiform encephalitis Chaga's disease

Human immuno deficiency virus (Retroviridae) Ascaris lumbricoides

Chickenpox

Chlamydial urethritis and pelvic inflammatory disease Cholera Cold sores

Common cold

Cow pox

Creutzfeldt-Jakob disease Cryptosporidiosis

Dengue fever

Diphtheria f-d

Aspergillus fumigatus, Aspergillus spp.

Epidermophyton spp., f-d Trichophyton spp.

Shigella dysenteriae, b-6g Shigella spp.

Clostridium b-7

botulinum

BSE prion pri

Trypanosoma cruzi p-f

Varicella-zoster virus v (Herpesviridae)

Chlamydia b-8

trachomatis

Vibrio cholerae b-6g

Herpes simplex v type 1 virus (Herpesviridae) Rhinoviruses v

(Picornaviridae), coronaviruses (Coronaviridae), adenoviruses (Adenoviridae), others

Vaccinia virus v

(Poxviridae)

CJD prion pri

Cryptosporidium p-s parvum

Dengue virus v (Flaviviridae)

Corynebacterium b-7 diphtheriae

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