Many infectious diseases of humans, including malaria, dengue, cholera, and schistosomiasis, are restricted to, or more prevalent in, tropical and subtropical zones. Within the tropics and subtropics, they are more prevalent at lower than at higher altitudes. Warmer temperatures characteristic of lower latitudes and altitudes generally increase rates of survival, development, and replication of parasites and of blood-feeding vectors such as mosquitoes. Warmer conditions also increase activity (including biting) rates of vectors, resulting in higher rates of parasite transmission (reviewed by Harvell et al. 2002). A series of papers in the 1990s (e.g., Shope 1991, Martens et al. 1995, Colwell 1996, McMichael et al. 1996, Patz et al. 1996) contended that recent and future trends in climate warming were likely to increase the incidence and geographic distribution of infectious diseases, particularly those caused by vector-borne and water-borne parasites and pathogens. Owing to media attention and popular concern, the spread of infectious diseases was featured in the growing list of negative outcomes known or anticipated to arise from anthropogenic climate change (e.g., Intergovernmental Panel on Climate Change 2001; the 2006 documentary film, An Inconvenient Truth). However, unequivocal demonstrations of a causal link between climate change and human infectious diseases are rare (albeit increasing). Some diseases are likely to decrease in incidence and range with climate warming (Harvell et al. 2002), and others are likely to respond to precipitation or humidity more than to temperature, leading to poor predictive power under warming scenarios. Many diseases are strongly influenced by other ecological, sociological, economic, and evolutionary factors besides climate change. These latter observations have stimulated the emergence of critics of a climate-change–infectious-disease linkage. Lafferty (2009) provides an overview of recent criticisms, emphasizing three major categories: (1) In many cases, we should expect diseases to shift geographically without net expansion under climate change; (2) non-climatic factors are more important than climate; and (3) models that predict increasing disease transmission with climate warming are flawed if transmission rates fail to exceed a specific threshold (R0 > 1) that allows disease persistence. Here, I discuss these criticisms in turn.