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"When reports emerged 30 years ago that young gay men were suffering from rare forms of pneumonia and cancer, public health investigators scrambled to understand what appeared to be a deadly immune disorder: What were the symptoms? Who was most susceptible? What kinds of infections were markers of the disease? They were seeking the epidemiologist's most essential tool - an accurate case definition, a set of criteria that simultaneously include people with the illness and exclude those without it. With AIDS, investigators soon recognized that injection-drug users, hemophiliacs and other demographic groups were also at risk, and the case definition evolved over time to incorporate lab evidence of immune dysfunction and other refinements based on scientific advances. "If you recognize something is happening, you need a case definition so you can count it," said Andrew Moss, an emeritus professor of epidemiology at the University of California, San Francisco, and an early AIDS investigator. "You need to know whether the numbers are going up or down, or whether treatment and prevention work. And if you have a bad case definition, then it's very difficult to figure out what's going on." Once a disease can be diagnosed reliably through lab tests, creating an accurate case definition becomes easier. But when an ailment has no known cause and its symptoms are subjective - as with chronic fatigue syndrome, fibromyalgia and other diseases whose characteristics and even existence have been contested - competing case definitions are almost inevitable. Now a new study of chronic fatigue syndrome has highlighted how competing case definitions can lead to an epidemiologic "Rashomon" - what you see depends on who's doing the looking - and has stoked a fierce debate among researchers and patient advocates on both sides of the Atlantic. "

In this edition I would like to explain why hot water freezes faster than cold water, when put into the freezer. It is a very counter-intuitive observation, it's a paradox. This is called the Mpemba Effect. The effect is named according to Tanzanian high-school student Erasto B. Mpemba who re-discovered the effect while making ice cream back in 1963. The Mpemba Effect is a nice example of how the change of one variable, the temperature, can have unexpected side effects. Most people assume that the difference between a hot glass of water and a cold glass of water is only the temperature. But this is not the case. Just by heating the water we are introducing a range of other variables that have an unexpected effect on the outcome.