*4.1. Principal Findings*

To the authors' knowledge, for the first time, an evaluation is reported on the research status and trend of climate change and infectious diseases around the world in the last two decades. The quantitative study clearly shows the exponentially rapid growth of the relevant publications, focusing on the topic of climate change and infectious diseases from 1999 to 2018. The articles published in the last decade (2009–2018) are nearly threefold of those in the previous decade (1999–2008).

According to the summarized distribution data of journals, the number of journals is almost twice as much in the last decade as in the previous decade, demonstrating that more and more journals began to publish articles regarding climate change and infectious diseases. It is also notable that comprehensive journals, for example, *Nature* and *PLoS One*, have shown interest in collecting papers in this field, confirming that the research on climate change-related infectious diseases had been supported by a wide range of subject categories. In the last two decades, however, the publication places of relevant journals were mainly developed countries or regions, such as The United States, England, Switzerland, and France.

As expected, this study provided some hints of the recent research hotspots in the field of climate change and infectious diseases. A co-word biclustering analysis was performed to reveal the research hotspots in the field, where there were four big clusters (nine subclass topics) and five big clusters (15) found in 1999 to 2008 and 2009 to 2018, respectively. Some topics, such as travel and tropic climate, global health, public health, environment, rain, biological model, and the greenhouse effect, have always been interesting for researchers. Human influenza, malaria, and emerging communicable diseases are the consistently concerned infectious diseases. From 1999 to 2008, hotspots like disasters, fever, and endemic diseases were not the same hotspots from 2009 to 2018. In the last ten years, researchers have begun to pay attention to some new hotspots, e.g., statistical model, diseases outbreaks, ecosystem, and biodiversity (Table 2 and Figure 5). Specifically, research on statistical models have become a hotspot, probably due to the development of data science, as well as new statistical methodology, such as deep learning, and their recent application in medicine [43]. Meanwhile, gastroenteritis; hand, foot, and mouth disease; diarrhea; dengue; and zoonoses are new foci among the research field of climate-sensitive communicable diseases. Hand, foot, and mouth disease, in particular, is a type of communicable disease emerging in recent decades. Although occurring much later in some countries like China, where it was originally seen in 1981, this disease is very prevalent, and often leads to outbreaks among children [44,45]. Therefore, researchers have begun to carry out related studies from multiple perspectives. In the aspect of its correlation with climate, a considerable amount of journal articles (32 articles) have been published within the last decade, according to the analysis of this study.

In parallel, the strategic diagram was employed to interpret the trends in themes during the two periods. Global health locates in Quadrant 1 all the time, indicating it is mature, but is indeed the eternal core theme of the whole body of literature. The topic on malaria is always in Quadrant 3, demonstrating that it is neither mature nor a core topic in the whole related field, and thus needs further investigation.

During the second decade (2009–2018), "Travel and tropical climate" within Quadrant 1, previously considered as the undeveloped and peripheral theme, progressed well and become

the core of the relevant field. New hotspots of "gastroenteritis and hand, foot, and mouth disease" and "disease outbreaks" have become mature in the last ten years, although they are still on the edge of the whole field. Other new hotspots, like "Statistical model", "Diarrhea", "Dengue", "Ecosystem and biodiversity", and "Zoonoses", are far from the research core, and do not connect closely with other subfield studies within the overall research network, and thus are neither mature nor the central topics in this field. They need more attention from researchers. However, research on some meteorological factors, such as "Greenhouse effect" and "Rain", remain unstable or undeveloped, and have shifted from the central to the edge of the whole research field.
