The Intergovernmental Panel on Climate (IPCC) has concluded that climate change has significant effects on human health. In order to understand how climate change impacts infectious diseases; it is essential to understand the pathways of disease transmission. Diseases transmitted directly from source of infection to the affected individual are least affected by environmental factors. If diseases are transmitted indirectly through a physical vehicle or a biological vector, climate change will have significant effect on disease transmission through the effect on the vehicle or the vector and the pathogen. The effect of climate change varies from one region to the other. Moreover, vulnerability to the effects of climate change varies from one individual to the other, from one group to the other and from one country to the other. Disease transmission is also affected by socio-economic factors, globalization of travel and trade, land use, unplanned urbanization, population growth, and cultural attitudes, among other factors.
Diseases transmitted via direct pathways include air-borne diseases like influenza, tuberculosis, measles or sexually transmitted diseases. Food-borne diseases and water-borne diseases like cholera and diarrhoeal diseases are transmitted indirectly, via physical vehicle.
Mosquitoes are the best known biological disease vectors. Diseases transmitted indirectly through mosquitoes can be transmitted either by mosquito-human-mosquito pathway such as malaria, dengue, yellow fever, chicungunia, zika and lymphatic filariasis; or by mosquito-animal-mosquito pathway such as west Nile virus disease, Rift valley fever and Saint Louis encephalitis.
Diseases transmitted indirectly via biological vectors also include tick-borne diseases such as lyme disease, tick-borne relapsing fever, typhus, Q fever and Rocky Mountain spotted fever; sandfly-borne diseases such as leishmaniasis; and rodent-borne diseases such as leptospirosis, hantavirus pulmonary syndrome, tularemia and plague through fleas.  Diseases can also be transmitted indirectly through other vectors like tse-tse fly, black fly, triatomine bug and aquatic snails.
Beginning in mid-1970s, there has been worldwide emergence, resurgence and redistribution of infectious diseases. Definitions of emerging diseases were given as well as examples of climate change impacts on emerging diseases. Examples given included west Nile virus disease, hantavirus pulmonary syndrome, Saint Louis encephalitis, Murray valley fever and Zika.