I live in America here a few people died from "west nile virus" (http://en.wikipedia.org/wiki/West_Nile_virus) which infects people through mosquitoes. In my home or in synagogue what can I do to get rid of (for health and bothering reasons) mosquitoes, spiders, flies, and roaches? Kill, catch and let go outside, push out the window, or something else? And source or reason please, Thank you very much

If the Department of Health, or other relevant departments, has issued a warning that the certain type of mosquito which carries the virus is found in your city - you are permitted to kill a mosquito which is around you. This is because there is safek pikuach nefesh, a doubt in a life-threatening case in which we are lenient. This doubt needs to be a real doubt and therefore, in this case, we require a warning of an organization concerned. It is important to note that in other cases the halacha might be different. If there is a spider or any other insect or animal which is life-threatening one is permitted to kill it. Even if this insect/animal is not currently posing a threat to him (i.e running after him). See Shulchan Aruch סימן שטז הלכה י, Shmirat Shabbat Khilchata פרק כה, א Spiders or animals which are not life-threatening but their bite or sting is very painful - best would be to capture them, even if they remain in their captured state over Shabbat. Second best would be to kill them by stepping on them but this must be done in a way so that others will not think you are permitted to kill on Shabbat. This means you can walk in the direction of the animal/insect and it happens to be stepped on whilst walking. See Shulchan Aruch סימו שטז י, Shmirat Shabbat כה ב Mosquitos and the like in a place where they do not carry viruses and therefore are not life-threatening but do bite or sting - can be 'shooed' away with your hands. If however, they are in the process of biting or stinging or even just sitting on your skin it is best to 'shoo' them away without touching them, but if this not possible you are permitted to capture them and let them go outside. But you are prohibited to kill them or throw them into a place where they will die for sure. Ants and Flies and the like which don't bite or sting one may spray close to them a spray which chases them away but does not kill them. Rabbi Baruch Kitay