For the sake of simplicity, let us start by naming an infectious agent a “germ.” There are countless germs that can infect human, animal, and plant hosts. Germs can be transmitted directly between hosts via respiratory air droplets or bodily fluids (e.g., saliva, blood, or secretions from sexual organs). Germs can also be transmitted indirectly through an intermediary source, for instance via mosquitoes, ticks, rodents, environmental particles (e.g., contaminated water and food) or contaminated blood products. Germs evolve and transform while new germs emerge regularly, implying their supply can be considered infinite. A broad distinction is often made between microscopically small germs with relatively short life spans, which replicate within their hosts (often called microparasites such as viruses, bacteria, and fungi), and much larger germs with relatively longer life spans (often called macroparasites such as parasitic worms). Many germs live inside or on the surface of their hosts’ bodies without causing illness or even discomfort.
Why This Book? An Introduction