Global climate and seasonal variations

• Temperature and precipitation are important drivers of vector-borne diseases and epidemics
• Many diseases whose epidemiology depends on warm, humid conditions, are endemic to the tropics and sub-tropics
• • Diseases influenced by climate often show seasonal patterns
  • Temperature determines rates of vector development and behaviour as well as viral replication
  • Also precipitation creates aquatic habitats such as ponds and stagnant pools, which allow insects and disease vectors to flourish and complete their life cycles
• Relief affects global patterns of disease because altitude causes abrupt changes in climate and disease habitats
• • Many diseases are water-borne
  • In the developing world millions of people rely on water from wells and surface supplies contaminated by sewage
  • Bacteria responsible for cholera and other infectious diseases thrive in these conditions
  • Unprotected and stagnant drinking water supplies also provide habitats for disease vectors
• • In the South Pacific, sustained temperatures of more than 32 degrees Celsius and humidity levels above 95% trigger warned of dengue epidemics
  • These conditions occur in the summer months, but short-term weather changes and exceptional rainfall events can also lead to outbreaks of the disease
• • In temperate regions in the northern hemisphere, epidemics of influenza peak in the winter months
  • Transmission of the flu virus is most efficient at lower temperatures (below 5 degrees Celsius) and when atmospheric humidity is low
  • These conditions occur most often in winter
• • In the tropics and sub-tropics, vector-borne diseases often reach a peak during the rainy season
  • For example, diarrhoeal disease in South Asia surges in the pre- monsoon and end of monsoon periods when fly populations are highest