Recent flooding in Australia (don't forget that it's summer there) has been followed by a spike in the number of Murray Valley encephalitis cases. MVEV is an arbovirus of flaviviridae (genus flavivirus, along with Yellow Fever, West Nile, and Dengue) and is transmitted by mosquitoes. In addition, MVEV persists in the bird population year round as a reservoir.
MVEV appears to cause no clinical presentation in the vast majority of infected individuals; however, a very small number of individuals develop encephalitis. According to the Health section of the New South Wales website, very young infants, travelers, and immigrants to Australia are at greatest risk of developing MVE, due to their lack of immunity and greater chance of becoming infected. Still, the vast majority of infected individuals exhibit no symptoms, and only a very small fraction present with MVE.
Some of you might remember the concerns over West Nile several years ago, when it was first spreading across the U.S. . While many infected individuals exhibited no symptoms, others would suffer from encephalitis and experience severe health problems. I'm not entirely sure what causes the discrepancy in how people react to the virus, but it might be an exciting topic for future reading.
Mosquito-borne brain-swelling virus on the rise (ABC Australia)
NSW Health Factsheet for MVEV