The Zika virus epidemic in the Western Hemisphere has been a tragedy. It appears to have sickened many babies with the condition microcephaly, which is characterized by babies being born with smaller heads. Babies with microcephaly are likely to experience developmental delays.
However, do we truly know that Zika is the cause of microcephaly? In the popular press and cable news, media simply state the virus is causing the disease. There is a clear correlation between the onset of this virus and the ensuing epidemic of microcephaly. But looking closer at the words of the directors of the CDC and the National Institute of Allergy and Infectious Disease, they use words such as “likely” and “more research is needed” to conclusively determine whether Zika is the cause or a correlation.
At what point can we say that a factor is truly a cause of a disease, whether that be cigarettes causing cancer or HIV causing AIDS? For infectious diseases, there are a historical set of principles called Koch’s Postulates, which help to structure the case for causation. Rigorous epidemiology can also help to build a case for causation. However, perhaps the most important criteria for proving causation is time, i.e. that the links between the disease and factor are reproduced in research by multiple researchers over time. With the progression of each of these lines of inquiry our confidence in causation can grow, and an improved understanding of the disease can lead to better diagnosis and treatments, now urgently needed.