Microbes may be the most significant life form sharing this planet with humans and plant because of their pervasive presence and their utilization of any available food source, including humans whose defenses may be breached. Microbial diseases are frequent and often severe, in Human Animal and Plant. The ubiquitous presence of microbes and their astronomic numbers give rise to the many mutants that account for rapid evolutionary adaptation and in part for emerging diseases and antibiotic-resistance. This adaptability also accounts for the ability of microbes to utilize an enormous range of nutritional sources. Depending on the food source, microbes may have either beneficial roles in maintaining life or undesirable roles in causing human, animals and plant disease.

Beneficial roles of microbes include recycling organic matter through microbe-induced decay and through digestion and nutrition in animals and humans. In addition, the natural microbial flora provides protection against more virulent microbes.

While microbes that cause infectious diseases are virulent, opportunistic diseases may also be caused by normally benign microbes. Opportunistic infections occur when the host defence mechanisms are impaired, when microbes are present in large numbers, or when microbes reach vulnerable body sites. A striking example is HIV, which impairs the host's defences against multiple microbes. Because death or severe impairment of an infected host compromises the survival of the infecting microbe, natural selection favours a predominance of less virulent microorganisms, except when microbial transmission depends on disease manifestations. Understanding and employing the principles of microbiology and the molecular mechanisms of pathogenesis enable physicians, medical scientists, and agricultural scientists to control an increasing number of infectious diseases our book will be beneficial to all students of agriculture, horticulture, and microbiology.