These infections are rare. Outbreaks of encephalitis attributable to these viruses are normally limited to specific geographic locations and seasons for several reasons. First, warm temperatures are normally required for the viruses to multiply and to be transmitted to a new host. Higher temperatures may quicken the transmission process and promote epidemic disease. However, the extent of this effect depends largely on the particular virus.
Some viruses require cooler weather and higher moisture conditions.
Thus, higher temperatures may reduce their prevalence. Second, environmental conditions that favor the presence of carriers and hosts must prevail. For example, relative humidity may affect plant life necessary for the feeding of hosts. Under those assumptions, infection of virtually the entire population of Miami was predicted to be possible unless protective measures were taken. An arbovirus is a virus transmitted by an arthropod.