Recovery From Infection: Exit of Microorganisms

RECOVERY FROM INFECTION: EXIT OF MICROORGANISMS

The primary requirement for recovery is that multiplication of the infective agent is brought under control, so that it ceases to spread around the body and that the damaging consequences of its presence are arrested and repaired. Such control is brought about by the combined functions of the phagocytic, immune and complement systems. A successful pathogen will not seriously debilitate its host; rather, the continued existence of the host must be ensured in order to maximize the dissemination of the pathogen within the host population. From the microorganism’s perspective the ideal situation is where it can persist permanently within the host and be constantly released to the environment. Although this is the case for a number of virus infections (chickenpox, herpes) and for some bacterial ones, it is not common. Generally, recovery from infection is accompanied by complete destruction of the organism and restoration of a sterile tissue. Alternatively, the organism might return to a commensal relationship with the host on the epithelial and skin surface.

Where the infective agent is an obligate pathogen, a means must exist for it to infect other individuals before its eradication from the host organism. The route of exit is commonly related to the original portal of entry (Figure 7.1). Thus, pathogens of the intestinal tract are liberated in the faeces and might easily contaminate food and drinking water. Infective agents of the respiratory tract might be exhaled during coughing, sneezing or talking, survive in the associated water droplets, and infect nearby individuals through inhalation. Infective agents transmitted by insect and animal vectors may be spread through those same vectors, the insects/animals having been themselves infected by the diseased host. For some ‘fragile’ organisms (e.g. N. gonorrhoeae, T. pallidum), direct contact transmission is the only means of spread between individual hosts. In these cases, intimate contact between epithelial membranes, such as occurs during sexual contact, is required for transfer to occur. For opportunist pathogens, such as those associated with wound infections, transfer is less important because the pathogenic role is minor. Rather, the natural habitat of the organism serves as a constant reservoir for infection.