Filarial worms infect over 140 million people worldwide and are transmitted by mosquitoes. They reside in lymphatic vessels and tissues. The most common forms that cause disease are Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, and Loa loa. They have complex lifecycles involving microfilariae that develop in mosquitoes before infecting humans. Lymphatic filariasis causes lymphatic damage and symptoms like hydrocele, lymphedema, and elephantiasis due to adult worms residing in lymphatics. Treatment involves diethylcarbamazine to kill microfilariae.