The geographical and seasonal distributions of many infectious diseases, particularly those that are vector borne, as well as those of many macroparasites and pests of various kinds are affected by climate. Pathogens, vectors, and intermediate and final hosts can all be affected both directly by the climate (e.g. temperature and humidity) and by the effects of climate on other aspects of their habitats (e.g. vegetation). If the climate changes, hosts and pathogens may be brought together in new locations and contexts, bringing new threats to animal health and new challenges for livestock management and policy.
         During recent decades, there have been significant changes in the incidence rates and/or distribution of several vector-borne diseases, including dengue, trypanosomiasis, leishmaniasis, Lyme disease, tick-borne encephalitis and bluetongue. Interpretation of observed changes and prediction of future trends in disease distribution and impact is complicated by numerous factors that act simultaneously, and interact, with climate change. Prominent among these interacting factors are changes to landscape and land cover, changes in the abundance of disease hosts including wildlife hosts, public health measures, trade, movements of human and animal populations, and a range of management, sociocultural, economic and political factors.
         Changes to seasonal climates can have a significant influence on disease distribution. In the case of many tropical diseases, it is the cold of winter that prevents them from becoming problems in temperate zones. Shorter or warmer winters may enable pathogens to become established in new areas. Seasonal changes can also affect disease epidemiology in more complex ways as the lifecycles of the various organisms that play, or potentially play, a role in disease transmission are affected. For example, there may be an increase in the temporal overlap of the active phases in the lifecycles of such organisms, providing new opportunities for disease transmission. Moreover, the various factors that affect the rate of transmission of vector-borne diseases may be simultaneously affected by changes to temperature, potentially leading to large increases in the impact of the disease. Beyond certain thresholds of temperature, faster rates of development may mean that pathogens are able to utilize additional species as vectors.
         Specific short-term weather events and/or seasonal rainfall patterns are known to be triggers for outbreaks of many diseases, including Rift Valley fever, African horse sickness, peste des petits ruminants, bluetongue and anthrax. It is predicted that climate change will lead to increases in the frequency of extreme weather events such as floods and droughts (IPCC). Flooding can contribute to the spread of waterborne diseases and diseases that are spread by vectors that have aquatic phases in their life cycles. Restrictions to water availability can also contribute to the spread of disease.

Rapid spread of pathogens, or even small spatial or seasonal changes in disease distribution, whether driven by climate change or not, may expose livestock populations to new disease challenges. The significance of this is not only that a larger number of animals may be exposed, but also that they are likely to lack the genetic resistance or acquired immunity that might have emerged had the animals or their ancestors previously been exposed to the diseases. The newly exposed populations may therefore experience more serious clinical disease. The problem may be confounded by a lack of experience on the part of the local livestock keepers in preventing and treating the unfamiliar diseases.
Animal genetic diversity potentially plays an important role in adapting production systems to the disease related effects of climate change, but this diversity may be in turn be threatened by these effects. There are several ways in which such threats may operate: large numbers of animals may die as a result of disease epidemics; large numbers of animals may be culled in disease-control programmes; disease pressure may be one among several factors that contribute to making livestockkeeping livelihoods unsustainable; and regulations brought in to combat disease may restrict the type of livestock keeping practised and thereby threaten the associated AnGR.

         The most severe recent epidemics in terms of the numbers of livestock lost – those in which millions or hundreds of thousands of animals have been culled or died from disease – have involved quite a narrow range of diseases: most notably foot-and-mouth disease, avian influenza, African swine fever, classical swine fever and contagious bovine pleuropneumonia. The extent to which these epidemics have affected AnGR diversity is not well understood.