In species that lay their eggs in rafts, rafts do not form adventitiously; the female Culex settles carefully on still water with its hind legs crossed, and as it lays the eggs one by one, it twitches to arrange them into a head-down array that sticks together to form the raft.Aedes females generally drop their eggs singly, much as Anopheles do, but not as a rule into water. Instead, they lay their eggs on damp mud or other surfaces near the waters edge. Such an oviposition site commonly is the wall of a cavity such as a hollow stump or a container such as a bucket or a discarded vehicle tire. The eggs generally do not hatch until they are flooded, and they may have to withstand considerable desiccation before that happens. They are not resistant to desiccation straight after oviposition, but must develop to a suitable degree first. After that, they can enter diapause for several months if they dry out. Clutches of eggs of the majority of mosquito species hatch as soon as possible, and all the eggs in the clutch hatch at much the same time. In contrast, a batch of Aedes eggs in diapause tends to hatch irregularly over an extended period of time. This makes it much more difficult to control such species than those mosquitoes whose larvae can be killed all together as they hatch. Some Anopheles species do also behave in such a manner, though not to the same degree of sophistication.
Dengue viruses are a significant health risk globally. Severe cases of dengue often require hospitalization and can be life-threatening shortly after infection. Symptoms include a high fever, aches and pains, vomiting, and rashes. Warning signs of severe dengue infection include vomiting blood, bleeding from the gums or nose, and stomach tenderness/pain. Equine encephalitis viruses, such as Eastern equine encephalitis virus, Western equine encephalitis virus, and Venezuelan equine encephalitis virus, can be spread by mosquito vectors such as Aedes taeniorhynchus. Tularemia, a bacterial disease caused by Francisella tularensis, is variously transmitted, including by biting flies. Culex and Culiseta are vectors of tularemia, as well as arbovirus infections such as West Nile virus. Zika, recently notorious, though rarely deadly, causes fever, joint pain, rashes and conjunctivitis. The most serious consequence appears when the infected person is a pregnant woman, since during pregnancy this virus can originate a birth defect called microcephaly. St. Louis Encephalitis, a mosquito-borne disease that is characterized by fever and headaches upon initial onset of infection, arises from mosquitoes who feed on birds who are infected with the illness, and can result in death. The most common vector of this disease is Culex pipiens, also known as the common house mosquito. Heartworm disease, a parasitic roundworm infection that affects dogs and other canids. Mosquitoes transmit larvae to the definitive host through bites.