Subtitle: "How Commerce Has Spread Disease". Which, frankly, is a bit of a bait-and-switch; it gives the impression that we are going to get a heretical look at the concept of free trade, which in modern times is sacrosanct to both progressives and liberals. Instead, we get a history of how free trade advocates have managed to subvert all efforts to halt the spread of disease.
In some ways, the problem that Harrison faces is that his chosen topic is chock full of data which is contrary to his stated beliefs. He is clearly sympathetic to free trade advocates (for most of human history, what were called "liberal", in the classical economic sense). However, when discussing the response of various nations and societies to bubonic plague, yellow fever, cholera, SARS, or swine flu, the facts seem to suggest that their attempts at quarantine were frankly ineffective for most of human history. The reason why is almost always that the merchant class (whether individual traders or large merchant houses) that chafed at such restrictions, and evaded them.
In some cases, with hindsight, we can say that one side or the other was correct. For example, we now know that plague was primarily spread by rats, not humans per se (although it was ships sailed by human traders who brought the rats). But at the time, there was great debate over whether or not plague was contagious. Advocates of free trade asserted that it was called by environmental conditions (not really true) or poor sanitation in the poorer regions of large cities (not exactly true, although clearly an aggravating factor). More conservative forces advocated keeping a strict quarantine, and thus the scientific question of whether or not plague (or yellow fever or cholera or...) was contagious or environmental, became a polital one.
My favorite character in this long debate is Doctor James McWilliam. He was a minor celebrity in England for having piloted a ship downstream from Niger when many of the crew, including the captain and engineer, had perished from disease (perhaps yellow fever). When a later ship (the "Eclair") had succumbed to yellow fever, and was accused by the Portugese of having brought it to the island of Boa Vista, Parliament picked McWilliam to investigate these accusations, relying upon him to understand what was required, and return a report saying that the disease that had struck the Eclair was not contagious, blame the (low ranking and young) captain of the ship for allowing insanitary conditions that caused the disease.
Unfortunately for Parliament, Dr. McWilliam took his job seriously, and interviewed many of the island's inhabitants, as well as surviving members of the crew. His report, when delivered to Parliament, acknowledged the role of proper hygiene and good medical care in resisting disease, but was clear that the British ship had brought an infectious disease to the Portugese island. For a Parliament that was looking for a medical report to bolster its case that no trade-reducing system of quarantine was necessary, it was an awkward bit of medical and scientific honesty. They quickly found a more accomodating naval doctor to deliver (apparently with much less research beforehand) a report whose conclusions they agreed with.
It is left to the reader to make comparison to modern day cases of the selection of medical or scientific authorities on the basis of the convenience of their conclusions, rather than the care with which they conducted their research. It is in different ways both comforting and discomforting that this phenomenon far predates our own age.
If there is one thing we cannot fault Harrison for, it is the care with which he researched his topic. It is exhaustively documented. We are often made privy to the debates of previous centuries, not only the policy conclusions which were drawn but the manner in which they were arrived at, and this is a perspective which is often missing from history.