PROLOGUE My First Parasite When I was little, my family lived in a crowded New York City neighborhood. Street after street of tall apartment houses stood side by side, stretching into the distance for miles. The streets echoed to the constant rumble of traffic. Luckily, Van Cortlandt Park lay within walking distance to our street. It was a two-mile walkm, but my friends and I went to the park often. It had a swimming pool, running track, tennis courts, and baseball fields. Best of all, it had a vast, wild area where we played "Jungle." We would crawl through the tangled brush or lie still in the tall grass, waiting to shoot each other with toy guns.

Once, as I got ready for bed after a long day of Jungle, I noticed several tiny brownish things clinging to my legs. They had hard shells, and felt like seeds. Yet, when I tried to brush them off, they did not budge. Instead, they seemed to tighten their grip with sharp tongs. It hurt. "Ticks," said the emergency-room doctor in the local hospital. "Your boy has ticks," he told my father, frowning. As the white-coated man spoke, he held up one of the tweezers he had used to remove it ever so gently.

This was my first meeting with a parasite. The doctor explained that ticks need to attach themselves to warm-blooded creatures like dogs, cats, squirrels--and people. Especially to little boys who play Jungle! Once attached, a tick bites the animal and sucks its blood. When it has taken its fill, the tick, swollen to three times normal size, drops to the ground. After digesting the blood, it will seek another animal for another meal of blood. Although I went on to become a professor of history, I have been interested in parasites ever since my run-in with those ticks. Over the years, I learned that everyone has parasites of one sort or another. Not only are parasites fascinating in thelmselves, they have helped shape human history for uncounted generations.

This book tells their story.