Filariasis is a major public health problem throughout many regions of the tropics. The disease is caused by several species of filarial nematode including Wuchereria bancrofti and Brugia malayi, the agents of lymphatic filariasis, and Onchocerca volvulus, the cause of 'riverblindness'. Disease caused by these worms varies depending on the tissue location of the parasite, and is associated with episodes of acute and chronic inflammation. These pathologies, including elephantiasis and blindness, rank among the most disabling in the world. Studies aimed at characterizing the molecular nature of the inflammatory stimuli derived from filarial nematodes uncovered a long forgotten secret, their symbiont Wolbachia. LPS-like molecules from these intracellular bacteria are responsible for potent inflammatory responses from macrophages and in animal models of filarial disease. Wolbachia has also been associated with severe inflammatory reactions to filarial chemotherapy, being released into the blood following the death of the parasite. Recent studies in animal models even implicate Wolbachia in the onset of lymphodema and blindness. Taken together these studies suggest a major role for Wolbachia in the pathogenesis of filarial disease. It may be possible, through the use of antibiotic therapy, to clear worms of their bacteria, in the hope that this will prevent the onset and development of filarial pathology.