ABSTRACT
Microbes are able to survive on animal and plant products by releasing digestive enzymes directly and absorbing the nutrients, and/or by growth on living tissues (extracellular), in which case they are simply bathed in nutrients. Other microbes infect (invade and live within) animal/human cells (intracellular), where they not only survive, but also replicate utilizing host-cell energy sources. Both extracellular and intracellular microbes can grow, reproduce, and infect other individuals. There are many different species of microbes and larger organisms (such as worms) that invade humans, some of which are relatively harmless and some even helpful (e.g., E. coli in the intestines). Many others cause disease (human pathogens). There is a constant battle between invading microbes and the immune system (Section H2). Although most pathogenic microbes do not cause mortality, they do produce global morbidity. Table 1 shows the range of organisms that can infect humans.
