Diseases That Can Be Transmitted From One Person to Another Are Termed

communicable diseases, in medicine, a process caused past an agent, oftentimes a type of microorganism, that impairs a person'southward health. In many cases, infectious affliction can be spread from person to person, either directly (due east.1000., via skin contact) or indirectly (due east.g., via contaminated food or h2o).

An infectious disease can differ from simple infection, which is the invasion of and replication in the torso by any of diverse agents—including leaner, viruses, fungi, protozoans, and worms—too as the reaction of tissues to their presence or to the toxins that they produce. When health is not altered, the process is called a subclinical infection. Thus, a person may be infected but non have an infectious disease. This principle is illustrated past the utilise of vaccines for the prevention of infectious diseases. For instance, a virus such as that which causes measles may exist attenuated (weakened) and used as an immunizing amanuensis. The immunization is designed to produce a measles infection in the recipient only mostly causes no discernible alteration in the state of wellness. It produces immunity to measles without producing a clinical illness (an infectious illness).

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The most important barriers to invasion of the human host by infectious agents are the peel and mucous membranes (the tissues that line the nose, mouth, and upper respiratory tract). When these tissues have been broken or afflicted by earlier disease, invasion by infectious agents may occur. These infectious agents may produce a local communicable diseases, such as boils, or may invade the bloodstream and exist carried throughout the body, producing generalized bloodstream infection (septicemia) or localized infection at a afar site, such as meningitis (an infection of the coverings of the encephalon and spinal cord). Infectious agents swallowed in food and drink tin can attack the wall of the abdominal tract and cause local or general disease. The conjunctiva, which covers the front of the eye, may be penetrated by viruses that cause a local inflammation of the eye or that pass into the bloodstream and cause a severe general disease, such as smallpox. Infectious agents can enter the torso through the genital tract, causing the acute inflammatory reaction of gonorrhea in the genital and pelvic organs or spreading out to assail almost whatever organ of the body with the more than chronic and destructive lesions of syphilis. Fifty-fifty before birth, viruses and other infectious agents can pass through the placenta and attack developing cells, so that an infant may be diseased or deformed at birth.

From conception to expiry, humans are targets for attack by multitudes of other living organisms, all of them competing for a identify in the common environment. The air people breathe, the soil they walk on, the waters and vegetation around them, the buildings they inhabit and work in, all tin can exist populated with forms of life that are potentially dangerous. Domestic animals may harbour organisms that are a threat, and wild animals teems with agents of infection that can afflict humans with serious affliction. However, the human body is non without defenses against these threats, for it is equipped with a comprehensive immune system that reacts rapidly and specifically against affliction organisms when they attack. Survival throughout the ages has depended largely on these reactions, which today are supplemented and strengthened through the employ of medical drugs.

Infectious agents

Categories of organisms

The agents of infection tin be divided into different groups on the basis of their size, biochemical characteristics, or fashion in which they collaborate with the human host. The groups of organisms that cause infectious diseases are categorized every bit leaner, viruses, fungi, and parasites.

Bacteria

Bacteria can survive within the torso but outside individual cells. Some bacteria, classified as aerobes, require oxygen for growth, while others, such as those normally found in the small intestine of healthy persons, abound only in the absence of oxygen and, therefore, are chosen anaerobes. Most bacteria are surrounded by a sheathing that appears to play an important role in their power to produce disease. Too, a number of bacterial species give off toxins that in plow may damage tissues. Bacteria are generally large enough to be seen nether a light microscope. Streptococci, the bacteria that cause scarlet fever, are nearly 0.75 micrometre (0.00003 inch) in diameter. The spirochetes, which cause syphilis, leptospirosis, and rat-bite fever, are five to 15 micrometres long. Bacterial infections can be treated with antibiotics.

Bacterial infections are normally caused by pneumococci, staphylococci, and streptococci, all of which are often commensals (that is, organisms living harmlessly on their hosts) in the upper respiratory tract only that can get virulent and crusade serious weather condition, such every bit pneumonia, septicemia (claret poisoning), and meningitis. The pneumococcus is the most common cause of lobar pneumonia, the disease in which 1 or more lobes, or segments, of the lung become solid and airless as a effect of inflammation. Staphylococci impact the lungs either in the course of staphylococcal septicemia—when bacteria in the circulating blood cause scattered abscesses in the lungs—or every bit a complication of a viral infection, commonly influenza—when these organisms invade the damaged lung cells and cause a life-threatening form of pneumonia. Streptococcal pneumonia is the to the lowest degree common of the three and occurs ordinarily as a complication of influenza or other lung disease.

Pneumococci ofttimes enter the bloodstream from inflamed lungs and cause septicemia, with connected fever but no other special symptoms. Staphylococci produce a type of septicemia with high spiking fever; the leaner can achieve almost whatsoever organ of the body—including the brain, the bones, and especially the lungs—and destructive abscesses form in the infected areas. Streptococci also cause septicemia with fever, simply the organisms tend to crusade inflammation of surface lining cells rather than abscesses—for example, pleurisy (inflammation of the chest lining) rather than lung abscess, and peritonitis (inflammation of the membrane lining the abdomen) rather than liver abscess. In the form of either of the last two forms of septicemia, organisms may enter the nervous arrangement and cause streptococcal or staphylococcal meningitis, but these are rare atmospheric condition. Pneumococci, on the other hand, often spread direct into the central nervous system, causing one of the common forms of meningitis.

Staphylococci and streptococci are mutual causes of skin diseases. Boils and impetigo (in which the skin is covered with blisters, pustules, and yellow crusts) may be caused past either. Staphylococci besides tin can cause a severe pare infection that strips the outer peel layers off the body and leaves the underlayers exposed, as in severe burns, a condition known every bit toxic epidermal necrolysis. Streptococcal organisms tin cause a severe condition known as necrotizing fasciitis, commonly referred to equally mankind-eating disease, which has a fatality rate between 25 and 75 percent. Streptococci tin can be the cause of the red cellulitis of the skin known as erysipelas.

Some staphylococci produce an abdominal toxin and cause food poisoning. Certain streptococci settling in the pharynx produce a reddening toxin that speeds through the bloodstream and produces the symptoms of scarlet fever. Streptococci and staphylococci also can cause toxic daze syndrome, a potentially fatal disease. Streptococcal toxic shock syndrome (STSS) is fatal in some 35 percent of cases.

Meningococci are adequately common inhabitants of the throat, in most cases causing no affliction at all. As the number of good for you carriers increases in any population, however, there is a tendency for the meningococcus to become more invasive. When an opportunity is presented, it can proceeds access to the bloodstream, invade the central nervous system, and cause meningococcal meningitis (formerly called cerebrospinal meningitis or spotted fever). Meningococcal meningitis, at one time a dreaded and still a very serious illness, usually responds to treatment with penicillin if diagnosed early enough. When meningococci invade the bloodstream, some gain access to the peel and cause bloodstained spots, or purpura. If the condition is diagnosed early plenty, antibiotics can clear the bloodstream of the bacterium and foreclose whatever from getting far plenty to crusade meningitis. Sometimes the septicemia takes a mild, chronic, relapsing grade with no tendency toward meningitis; this is curable once it is diagnosed. The meningococcus also can cause one of the most fulminating of all forms of septicemia, meningococcemia, in which the body is rapidly covered with a royal rash, purpura fulminans; in this form the blood pressure becomes dangerously low, the heart and claret vessels are afflicted past daze, and the infected person dies within a matter of hours. Few are saved, despite treatment with appropriate drugs.

Haemophilus influenzae is a microorganism named for its occurrence in the sputum of patients with influenza—an occurrence so common that it was at in one case idea to be the cause of the disease. It is now known to be a mutual inhabitant of the nose and throat that may invade the bloodstream, producing meningitis, pneumonia, and various other diseases. In children it is the nigh mutual cause of acute epiglottitis, an infection in which tissue at the back of the tongue becomes chop-chop swollen and obstructs the airway, creating a potentially fatal status. H. influenzae also is the nigh common crusade of meningitis and pneumonia in children nether five years of historic period, and it is known to cause bronchitis in adults. The diagnosis is established by cultures of blood, cerebrospinal fluid, or other tissue from sites of infection. Antibiotic therapy is generally effective, although death from sepsis or meningitis is nonetheless mutual. In adult countries where H. influenza vaccine is used, in that location has been a not bad decrease in serious infections and deaths.

Chlamydial organisms

Chlamydia are intracellular organisms institute in many vertebrates, including birds and humans and other mammals. Clinical illnesses are caused by the species C. trachomatis, which is a frequent cause of genital infections in women. If an infant passes through an infected birth culvert, it can produce disease of the center (conjunctivitis) and pneumonia in the newborn. Young children sometimes develop ear infections, laryngitis, and upper respiratory tract disease from Chlamydia. Such infections can be treated with erythromycin.

Another chlamydial organism, Chlamydophila psittaci, produces psittacosis, a disease that results from exposure to the discharges of infected birds. The illness is characterized past high fever with chills, a slow center rate, pneumonia, headache, weakness, fatigue, muscle pains, anorexia, nausea, and vomiting. The diagnosis is unremarkably suspected if the patient has a history of exposure to birds. Information technology is confirmed by blood tests. Mortality is rare, and specific antibody treatment is available.

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Source: https://www.britannica.com/science/infectious-disease

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