Thus, in a few years, from experiments on mosquitoes, sparrows, and men, has come the present plan of campaign against malaria, yellow fever, and filariasis; that is, against _Anopheles_ and _Culex_. He who would know what is being done to check these diseases in Italy, India, China, Africa, and America, must read Prof. Ross' _Malarial Fever, its Cause, Prevention, and Treatment_ (1902), and _Mosquito Brigades, and how to organise them_ (1902). There has been nothing like it since Pasteur died. Far and wide, from Staten Island to Cuba, from Hong Kong to Lagos, the work of keeping down the larvæ of _Anopheles_ and _Culex_ is going on. _Henceforth we have to reckon not with a nameless something, but with a definite parasite, whose conditions of life are known. Before all things, we must shut off the sources of the infection._ For centuries, men had believed in exhalations and miasmata lying all over the land: and, behold, the agents of malaria are in the puddles round a man's house, and the agents of yellow fever are in the water-butt and the broken bottles and old sardine-tins. Science has given the word, and now there are _Anopheles_ brigades and _Culex_ brigades set going; labourers with brooms and rubbish-carts, sweeping out the stagnant pools, draining the surface soil, and carrying off the odd receptacles that serve to hold mosquito eggs and larvæ. The job, like all sanitary jobs, must be steady, year in, year out: it must be limited to infected places, a whole continent cannot be treated. But there the work is, and will grow; and saves, by unskilled labour, and at a trivial expense, those "non-acclimatised" lives that have hitherto been thrown away as recklessly as the larvæ that are now swept out of the puddles and ditches round African settlements. XI PARASITIC DISEASES The foregoing chapters are concerned with bacteriology alone, and with those curative or preventive methods of treatment that have come out of inoculation-experiments on animals. The lives that are saved, or safeguarded, by these methods, even in one year, must be many thousands in each country of the civilised world. And, beside human lives, there is the protection of sheep and cattle against anthrax, swine against rouget, horses against tetanus, cattle against rinderpest. In Cape Colony alone, so far back as 1899, about half a million cattle had received preventive treatment against rinderpest; and the sum total of human and animal lives saved or safeguarded, in all parts of the world, must be reckoned in millions by this time. The present chapter, and the next two chapters, are concerned with methods that have come out of experiments on animals, but not out of bacteriology.