Contagion: How Commerce Has Spread Disease.

By Mark Harrison. Yale University Press; 376 pages; $38.

Spillover: Animal Infections and the Next Human Pandemic. By David Quammen. W.W. Norton; 592 pages; $28.95.

On Oct. 2, a British traveler flying home to Glasgow from Afghanistan began to feel ill. Within hours he was diagnosed with Crimean-Congo hemorrhagic fever. Less than 24 hours later he was dead.

This outbreak, on top of another death last month in Saudi Arabia from a previously unknown virus, has set global health agencies on edge. Ten years ago the deaths of a couple of travelers from foreign parts might not have been news at all. But the fright of the SARS outbreak in 2003 has left a lasting impression, and scientists and public-health officials now tend to see any disease threat through its lens.

It is refreshing, therefore, to take a wider look at the problem of infectious disease. Two recent books take very different approaches to the narrative of bacteria and viruses that humanity has known for centuries and the brand-new bugs that, by opportunistic accident, hop between species and start a new evolutionary tussle.

Mark Harrison, director of Oxford University's Wellcome Unit for the History of Medicine, charts a chronological path through the history of such diseases. David Quammen, an American science journalist, picks up the story of contemporary blights, exploring how the next pandemic will be detected.

Quammen focuses on infections that pass from animals to humans -- nearly two-thirds of all human infectious diseases. Three recent outbreaks -- SARS, bird flu and swine flu -- indicate that the next pandemic is likely to come from animals.

Quammen analyzes individual diseases, searching for patterns in their outbreaks.