Step into the hidden world of microbes and learn the challenges and advantages of being small—very small. Microbes live in a realm where water seems as thick as molasses and the smoothest surface conceals a canyon of hiding places. Also see how the geometry of a sphere explains how bacteria survive.

Turn back the clock to a time when our early ancestors escaped most epidemic diseases. But once we started gathering into villages, raising crops, and domesticating animals, we changed our niche and altered our habitat. Deadly microbes thrived in these new conditions.

Follow the trail of one of the most infamous microbes of all time, Yersinia pestis, the cause of the Black Death. Like typhus, malaria, and dengue fever, the Black Death is a vector-borne disease—one transmitted from human to human via a host intermediary; in this case, fleas.

In the days before the invention of the microscope and the rise of modern medicine, how did people explain a killer plague? Retrace the steps that led pioneers such as Louis Pasteur, Robert Koch, and Ignaz Semmelweis to the startling conclusion that organisms invisible to the naked eye cause disease.

In the first of three lectures on the coevolution that shapes our relationship with the microbial world, explore the discovery of antibiotics and the subsequent upsurge in antibiotic-resistant strains of bacteria, driven by our overuse of drugs that were once a magic bullet against infection.

Probe the different mechanisms that humans have evolved to defeat microbial invaders, and the strategies evolved by microbes to thwart those defenses. For example, our immune system is primed to produce fever and other infection-fighting responses, but many microbes have developed frighteningly potent countermeasures.

Virulence is a measure of the effectiveness of a microorganism at killing its victims. Discover that many diseases, such as syphilis, scarlet fever, and diphtheria, have grown less virulent due to competition and coevolution. On the other hand, vector-borne pathogens often succeed by growing more virulent.

Chart the human-created niches where microbes flourish. Trade, travel, and technological innovations provide new opportunities for the evolution or dispersal of pathogens, including Legionnaires’ disease in air conditioning systems, toxoplasmosis in kitty litter, and Oropouche fever in fields cleared for the cultivation of cacao, used in making chocolate.

Consider more examples of how ecological disturbances, both natural and human-made, can benefit harmful microbes. Thanks to land-clearing and the subsequent explosion in the deer population, Lyme disease now occurs throughout much of the United States. More frightening and deadly, if less widespread, are hantavirus, Lassa fever, and Ebola.

The hookworm influenced an early 20th-century stereotype of Southerners as indolent and undernourished, and it may have contributed to the outcome of the Civil War. Chart the war waged against this debilitating parasite by zoologist Charles Wardell Stiles, whose public health crusade helped transform the South.

In the first of three lectures on the deadliest epidemic of all time, meet the virus that caused the 1918 flu, investigating its structure, method of infection, and strategy for evading the human immune system. Also learn where it first appeared and how it mutated into a far more virulent strain.

Track the mutated form of the 1918 flu as it reached American shores and killed an estimated 675,000 people out of a population of 105 million. Philadelphia is a horrifying example of the medieval-like conditions that affected a bustling city trying to deal with mass infection and death.

Follow one of the most gripping detective stories of modern times—the search to recover an intact virus from the 1918 flu. Also learn what made the 1918 flu a more powerful killer than the similar strain that attacked in 1976 and 2009.

Given the proliferation of microbes in our midst, why aren’t we sick all the time? In the first of six lectures on the inner mysteries of the immune system, see how different cells have evolved to distinguish self from non-self, providing the first line of defense against infection.

Delve deeper into the mechanics of adaptive immunity to learn how a few hundred genes can easily make more than 100 million different antigen receptors, specific to any foreign invader that enters the body. Also discover the crucial difference between resistance and immunity.

In our age-old struggle with microbes, have we finally met our match with AIDS? The HIV virus that causes AIDS takes aim at the very heart of the human immune system. Probe this elegant strategy and learn where and when HIV first appeared, and why it is so lethal.

Explore the frightening scenarios that may yet unfold with the AIDS pandemic. Then follow the slow progress in developing an AIDS vaccine, and consider the policy of deferring questions of sexual morality to focus on preventing spread of the virus at all costs.

Consider what happens when the immune system turns on us, attacking our own cells and tissues as if we were the enemy. Such autoimmune diseases include multiple sclerosis, rheumatoid arthritis, Type 1 diabetes, and lupus. Examine the mysterious causes of this self-destructive reaction.

In the closing lecture on the human immune system, follow the microscopic chain of events that lead to allergies and asthma. Peanuts, pollen, bee stings, cat hair—all can cause an overreaction in the immune system, but for different reasons and with results that range from discomfort to death.

Investigate the history of microbes as weapons, which dates to the practice of catapulting disease-infected corpses into enemy strongholds. Germ warfare was even used during the American Revolutionary War, but it didn’t reach maturity until World War II with Unit 731, the notorious project run by the Japanese.

As if from Pandora’s box, the technology of germ warfare advanced during the cold war to a lethality rivaled only by atomic weapons. Draw back the curtain on the secret American and Soviet projects that perfected this weapon, and learn why biological warfare is the strategy of choice for terrorists.

When European explorers arrived in the New World, they unwittingly brought weapons far more lethal than firearms: namely, microbes, such as smallpox, that the Indians had never encountered. Learn why diseases bred through contact with domesticated animals in the Old World swept through the Americas like the angel of death.

Is there life beyond Earth? Space is filled with the chemicals essential for life, but so far only indirect evidence for possible microbial life has been found. Also, look at the microbes that thrive in extreme environments on Earth that may resemble conditions on other worlds.

In this last lecture, consider how the vast majority of microbes are harmless or even beneficial to humans. Microorganisms are responsible for everything from the oxygen in air to yogurt and many medicines. They may even help us clean up our planet, proving that the microscopic world is not always the stuff of nightmares!