COURSE OF INFECTION
The course of an infection can be considered to follow up to four major stages namely: adhesion, entry, localized infection and generalized infection.
A virulent pathogen is one that is well adapted to establish an infection. Most pathogens are adapted to adhere to cells, usually epithelial, that line the site of entry. Adherence is the first stage of infection, given that it prevents the pathogen being swept away and eliminated from the body.
Pathogens enter the body through one of a number of so-called portals of entry. These include the skin and conjunctiva, respiratory tract, gastrointestinal tract (GIT), urogenital system and, in the case of fetuses, the placenta. Once entry has been gained, conditions for growth, such as temperature, nutrients, must be favorable, but the pathogen must also be able to overcome the local defenses. Pathogens are often adapted to enter their hosts through a single portal of entry and do not cause infectious diseases if they enter through a different portal.
A localized infection acts as a focus of infection and, indeed, many remain local in nature or are prevented from spreading by host defenses. If the pathogenic organisms penetrate tissues and reach the blood or lymphatic systems or enter cells such as phagocytes, they can be distributed throughout the body and infect other tissues and organs causing a generalized infection. Again, certain pathogens are particularly well adapted to infect specific target tissues and organs. During infections, the pathogen multiplies and can be shed from the host into the environment through portals of exit. In localized infections these are the same as that of entry; in a generalized infection, other portals are used. These may include loss from the skin in pus, scales and blood, from the respiratory tract as droplets and aerosols of mucus and saliva, from the GIT in feces and bile, from the urogenital system in urine, mucus and genital secretions and, in pregnancy, from the placenta with direct transfer to the fetus.
A pathogen may be eliminated at any stage of the infective process. In some cases, the growth may be held in check but not eliminated. Such latent infections can be activated later giving recurring infections. In some cases, individuals can recover from a disease but the pathogen may remain in the body for considerable periods. Such people are symptomless carriers and are reservoirs of the disease.
Some viruses which exhibit latency, with or without sporadic reactivation are shown in Table 2.2.
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