We live in a unique time in human history. For a starter, so many of us. At the end of 2012 we hit 7 billion. And just keep adding more with one birth every eight seconds. To put this into perspective, I was born in 1974. I was 3,972,326,007th person alive at the time. By the time my 50th birthday rolls around in 2024, there will be 8 billion people. The world's population will have doubled during my lifetime. Another way the present is so unique, is where we live. For the first time in human history, more than half of us live in cities. And, finally, if you live in America, you spend over 90% of your life indoors. Likely the same in most other developed countries. These are unique times. It wasn't always like this of course. We evolved in the plains of Africa, spread out across the colder reaches of the world where we lived in caves, and began to settle down into small communities 10,000 years ago as we developed agriculture. And eventually cities between five and 3,000 years ago. As we changed the way we live, we also changed the ways diseases are spread. We have always had infectious diseases, of course. And we know the types from different lines of evidence. We can examine worms and fossilized feces from 12,000 year old cave dwellings in North America, deduce the tell tale signs of TB infection in the bones of Egyptian pharaohs, or read the written records by Thucydides of smallpox-like infections that ravaged the Athenian army during the Peloponnesian War in 431 BC. But because this Modern Age really is special, then in some cases we have created truly unique ways for diseases to spread. We are just now collecting the evidence and finding some impressive surprises. Such a case is the SARS epidemic of 2003. SARS is a virus in the same group as the common cold, cornonviruses. Unlike the common cold, it kills 10% of the people it infects. And in 2003, spread across 30 countries and regions. Infecting 8,273 people. Killing 775 people. It began in Guangdong Province on mainland China. Likely coming from food markets where people trade wild animals such as civets and bats which to eat. That is not novel. 60% of human disease comes from the animals we eat. What is novel is what happened next. In Hong Kong, the disease arrived with a traveler from the mainland. Cases began being reported from Amoy Gardens, a massive 33 story housing complex with 19,000 inhabitants. In total 329 residents became infected with 42 deaths reported. Importantly, only 4% were infected from direct contact with another infected person. How then did it spread? In a stunning investigation by the Hong Kong Department of Health, collaborated with eight governmental agencies, the truth emerged. The index case was a 33 year old man with end stage kidney disease who came from Shenzhen, China to Hong Kong for treatment. He stayed with his brother in Amoy gardens, and for a few days, before leaving again. When people started becoming sick, it was clear there was a strong spatial component. The Amoy gardens has 19 blocks, and 49% of cases were in block E, with the majority of the remaining cases in nearby blocks. Although the index case stayed in block E, he did not interact directly with the other people. They did, however, share a sewage system. One important feature of the Amoy Gardens infection was severe watery diarrhea, with 73% of cases having this unpleasant symptom an average of 3.9 times a day. The way many 1980s buildings in Hong Kong were constructed was with a vertical shaft where all waste from thirty to fifty floors would pass down a single tube. This was the case in Amoy Gardens. Also, in each bathroom, there was a floor drain that linked the air in the bathroom to the same vertical shaft. The virus was able to pass from the vertical shaft with SARS-infected diarrhea to the bathrooms of other apartments. This happened not passively, but when occupants closed the bathroom door and turned on the extraction fan, thereby creating a negative pressure, sucking the virus into their homes. The floor drain should have had a water trap. This is a small reservoir of water in the pipes that would have acted as a barrier, preventing viral particles entering the bathroom. Unfortunately, because the buildings are so high, there is a massive pressure int he pipes, which literally blew the water drops away allowing the virus to travel to the bathrooms and then the human lungs. Technical plumbing details of the sewer proved vital clues in the investigation. SARS is a sobering lesson of the very unexpected ways our modern cities can drive disease transmission. At no time in human history have we lived in apartment buildings with 19,000 residents, and shared air and sewage supplies with them. While the cost of the epidemic was high, it could've been much worse. What we need to do now is learn lessons of that event, and design better spaces where, for better or worse, we have decided to spend so much of our time.