For each pathogen the primary interest is whether

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For each pathogen, the primary interest is whether there is natural immunity and, if so, whether it is based on cell-mediated (T H 1, CMI) or humoral (T H 2, antibody) mechanisms. Humoral and CMI responses are broadly stimulated with most infections, but the specific response to a particular molecular structure is usually dominant in mediating immunity to reinfection. For example, the repeated nature of strep throat (group A streptococcus) in childhood is not due to antigenic variation as described for gonorrhea. The antigen against which protective antibodies are directed (M protein) is stable but naturally exists in over 80 types. Each type requires its own specific antibody. Knowing the molecule against which the protective immune response is directed is particularly important for devising preventive vaccines. Clinical Aspects of Infectious Disease Manifestations Fever, pain, and swelling are the universal signs of infection. Beyond this, the particular organs involved and the speed of the process dominate the signs and symptoms of disease. Cough, diarrhea, and mental confusion represent disruption of three different body systems. On the basis of clinical experience, physicians have become familiar with the range of behavior of the major pathogens. However, signs and symptoms overlap considerably. Skilled physicians use this knowledge to begin a deductive process leading to a list of suspected pathogens and a strategy to make a specific diagnosis and provide patient care. Through the probability assessment, an understanding of how the diseases work is a distinct advantage in making the correct decisions. Diagnosis A major difference between infectious and other diseases is that the probabilities just described can be specifically resolved, often overnight. Most microorganisms can be isolated from the patient, grown in artificial culture, and identified. Others can be seen microscopically or detected by measuring the host- specific immune response. Preferred modalities for diagnosis of each agent have been developed and are available in clinic, hospital, and public health laboratories all over the world. Empiric diagnosis made on the basis of clinical findings can be confirmed and the treatment plan modified accordingly. The new molecular methods, which detect molecular structures or genes of the agent, are not yet practical for most infectious diseases. Treatment Over the last 70 years, therapeutic tools of remarkable potency and specificity have become available for the treatment of bacterial infections. These include all the antibiotics and an array of synthetic chemicals that kill or inhibit the infecting organism, but have minimal or acceptable toxicity for the host. Antibacterial agents exploit the structural and metabolic differences between bacterial and eukaryotic cells to provide the selectivity necessary for good antimicrobial therapy. Penicillin, for example, interferes with the synthesis of the bacterial cell wall, a structure that has no analog in human cells. There
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