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tweena sleeper and host-seeking mosquitoes. Thus a chemical barrier is added to the often incompletephysical barrier provided by the net. Treated nets may be considered as mosquito traps baitedby the odour of the sleeper. Trials in Assam, Tanzania and elsewhere have shown that when awhole community is provided with treated nets, so many mosquitoes of anthropophilic species arekilled by contact with the nets that the density and/or sporozoite rate of the vector population is reduced.In order to gain this “mass” or community effect, in addition to widespread personal protection,and thus to achieve the full potential of the treated net method, a high per cent coverage ofthe community is needed. This suggests that organised free provision of treated nets, comparableto a house spraying programme, is likely to be more cost-effective than trying to market nets andinsecticide to very poor rural people. In areas with high malaria transmission, where acquisition ofimmunity to malaria is very important, it has been argued that vector control (without vector eradication)could, in the long run, make the situation worse by preventing the normal build-up of immunity.However, our data from Tanzania do not support this idea—3–4 years after provision of nets(which are re-treated annually) young children are still showing clear health benefits; older childrenare not “paying” for this by showing worse impact of malaria. There is less malaria morbidity in ahighland area where malaria transmission is about 15x less intense than in a nearby lowland area.The per cent impact of treated nets malaria morbidity in both areas was very similar. At presentonly pyrethroids are used for net treatment which suggested that emergence of pyrethroidresistance would have a disastrous effect. However, in West Africa, where there is now a highfrequency of the kdr resistance gene in Anopheles gambiae, it is reported that treated nets continueto have a powerful impact on vector populations. In Tanzania, pyrethroid resistance has notbeen detected in malaria vectors, but it has emerged in bedbugs after seven years use of treatednets.