Many people think of bacteria, viruses, and other microorganisms as our enemy.
This perspective, called the germ theory of disease, was first proposed in the 19th century. It maintained that illness springs from the actions of infecting microorganisms, drawing battle lines between “us” (the afflicted hosts
) and “them” (the invading microbes). Germ theory was crucial in identifying many diseases caused by microbes and in finding ways to prevent them through measures such as immunization
, sanitation, and improved living conditions. By the mid-1960s many experts concluded that infectious disease was all but conquered and researchers could shift their focus to chronic
medical conditions such as heart disease and cancer.
Today, many scientists recognize the need for a more ecological view of the microbial world around us: Microbes and their hosts (including humans) ultimately depend on each other for survival.
But this optimism was shaken in the mid-1970s and early 1980s with the appearance of Lyme disease
, Legionnaires’ disease, toxic shock syndrome, and human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS
). It was dealt further blows with the development of antibiotic-resistant
bacteria and the appearance and spread of severe acute respiratory syndrome (SARS
), H1N1, Middle East respiratory syndrome coronavirus (MERS-CoV), and Ebola
. Scientists began to re-examine the relationship between hosts and microbes.
Today, many scientists recognize the need for a more ecological
view of the microbial world around us: Microbes and their hosts (including humans) ultimately depend on each other for survival. And although the microorganisms that cause disease often receive more attention, most microorganisms do not cause illness. In fact, many of them protect us, helping our bodies function properly and competing with harmful organisms in an ongoing contest for habitable space in and on our bodies.
There is a close connection between microbes and humans. Experts believe that about half of all human DNA
originated from viruses that infected and embedded their nucleic acid in our ancestors’ egg and sperm cells. Microbes occupy all of our body surfaces, including the skin, gut, and mucous membranes
. In fact, our bodies contain at least 10 times more bacterial cells than human ones, blurring the line between where microbes end and humans begin. Microbes in the human gastrointestinal tract
alone comprise at least 10 trillion organisms, representing more than 1,000 species, which are thought to prevent the gut from being colonized by disease-causing organisms. Among their other beneficial roles, microbes synthesize vitamins, break down food into absorbable nutrients, and stimulate our immune system
The vast majority of microbes establish themselves as persistent “colonists,” thriving in complex communities within and on our bodies. In many cases, the microbes derive benefits without harming us; in other cases, both host and microbe benefit. And though some microbes make us sick and even kill us, in the long run they have a shared interest in our survival. For these tiny inhabitants, a dead host is a dead end.
The success of microorganisms is due to their remarkable adaptability. Through natural selection, organisms that are genetically better suited to their surroundings have more offspring and transmit their desirable traits to future generations. This process operates far more rapidly in the microbial world than in people. Humans produce a new generation every 20 years or so; bacteria do it every 20 to 30 minutes, and viruses even faster. Because they reproduce so quickly, microorganisms can assemble in enormous numbers with great variety in their communities. If their environment suddenly changes, the community’s genetic variations make it more likely that some will survive. This gives microbes a huge advantage over humans when it comes to adapting for survival.