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A pathological condition spread among biological species. Infectious diseases, although varied in their effects, are always associated with viruses, bacteria, fungi, protozoa, multicellular parasites and aberrant proteins known as prions. A complex series of steps, mediated by factors contributed by both the infectious agent and the host, is required for microorganisms or prions to establish an infection or disease. Worldwide, infectious diseases are the third leading cause of human death.
The most common relationship between a host and a microorganism is a commensal one, in which advantages exist for both organisms. For example, hundreds of billions of bacteria of many genera live in the human gastrointestinal tract, coexisting in ecological balance without causing disease. These bacteria help prevent the invasion of the host by more virulent organisms. In exchange, the host provides an environment in which harmless bacteria can readily receive nutrients. There are very few microorganisms that cause disease every time they encounter a host. Instead, many factors of both host and microbial origin are involved in infectious disease. These factors include the general health of the host, previous exposure of the host to the microorganism, and the complement of molecules produced by the bacteria.
Spread of a pathogenic microorganism among individual hosts is the hallmark of an infectious disease. This process, known as transmission, may occur through four major pathways: contact with the microorganism, airborne inhalation, through a common vehicle such as blood, or by vector-borne spread.
The manner in which an infectious disease develops, or its pathogenesis, usually follows a consistent pattern. To initiate an infection, there must be a physical encounter as which the microorganism enters the host. The most frequent portals of entry are the respiratory, gastrointestinal, and genitourinary tracts as well as breaks in the skin. Surface components on the invading organism determine its ability to adhere and establish a primary site of infection. The cellular specificity of adherence of microorganisms often limits the range of susceptible hosts. For example, although measles and distemper viruses are closely related, dogs do not get measles and humans do not get distemper. From the initial site of infection, microorganisms may directly invade further into tissues or travel through the blood or lymphatic system to other organs.
Microorganisms produce toxins that can cause tissue destruction at the site of infection, can damage cells throughout the host, or can interfere with the normal metabolism of the host. The damage that microorganisms cause is directly related to the toxins they produce. Toxins are varied in their mechanism of action and host targets. See Cholera, Staphylococcus
The host's reaction to an infecting organism is the inflammatory response, the body's most important internal defense mechanism. Although the inflammatory response is also seen as secondary to physical injury and nonspecific immune reactions, it is a reliable indicator of the presence of pathogenic microorganisms. Immune cells known as lymphocytes and granulocytes are carried by the blood to the site of infection. These cells either engulf and kill, or secrete substances which inhibit and neutralize, microorganisms. Other white blood cells, primarily monocytes, recognize foreign organisms and transmit chemical signals to other cells of the host's immune system, triggering the production of specific antibodies or specialized killer cells, both of which are lethal to the infecting microorganism. Any influence that reduces the immune system's ability to respond to foreign invasion, such as radiation therapy, chemotherapy, or destruction of immune cells by an immunodeficiency virus such as HIV, increases the likelihood that a organism will cause disease within the host.
Chemical compounds that are more toxic to microorganisms than to the host are commonly employed in the prevention and treatment of infectious disease; however, the emergence of drug-resistant organisms has led to increases in the morbidity and mortality associated with some infections. Other methods for controlling the spread of infectious diseases are accomplished by breaking a link in the chain of transmission between the host, microorganism, and mode of spread by altering the defensive capability of the host. Overall, the three most important advances to extend human life are clean water, vaccination, and antibiotics (in that order of importance).
Water-borne infections are controlled by filtration and chlorination of municipal water supplies. Checking food handlers for disease, refrigeration, proper cooking, and eliminating rodent and insect infestation have markedly reduced the level of food poisonings. The transmission of vector-borne diseases can be controlled by eradication of the vector. Blood-borne infections are reduced by screening donated blood for antibodies specific for HIV and other viruses and by rejecting donations from high-risk donors. For diseases such as tuberculosis, the airborne spread of the causative agent, Mycobacterium tuberculosis, can be reduced by quarantining infected individuals. The spread of sexually transmitted diseases, including AIDS, syphilis, and herpes simplex, can be prevented by inhibiting direct contact between the pathogenic microorganism and uninfected hosts. See Acquired immune deficiency syndrome (AIDS), Food poisoning, Vaccination, Water-borne disease