I can think of a few microorganisms, possibly the tubercle bacillus, the syphilis spirochete, the malarial parasite, and a few others, that have a selective advantage in their ability to infect human beings, but there is nothing to be gained, in an evolutionary sense, by the capacity to cause illness or death. Pathogenicity may be something of a disadvantage for most microbes, carrying lethal risks far more frightening to them than to us. The man who catches a meningococcus is in considerably less danger for his life, even without chemotherapy, than meningococci with the bad luck to catch a man.
Formerly character proved a strong curb for passions; in the present there is not much strength in character, and it grows less and less because of the prevailing scepticism, which is a decomposing element. It is like a bacillus breeding in the human soul; it destroys the resistant power against the physiological craving of the nerves, of nerves diseased. The modern man is conscious of everything, and cannot find a remedy against anything.henryk sienkiewicz
Man is a noisome bacillus whom Our Heavenly Father created because he was disappointed in the monkey.Bernard DeVoto