The beginning of our knowledge of the cause dates from 1851, when small rod-shaped bodies (Fig. 17) were found in the blood of the affected cattle, and by the work of a number of observers it was established that these bodies were constantly present.  Nothing was known of their nature; some held that they were living organisms, others that they were formed in the body as a result of the disease.  Next the causal relation of these bodies with the disease was shown and in several ways.  The disease could be caused in other cattle by injecting blood containing the rods beneath the skin, certainly no proof, for the blood might have contained in addition to the rods something which was the real cause of the disease.  Next it was shown that the blood of the unborn calf of a cow who died of the disease did not contain the rods, and the disease could not be produced by inoculating with the calf’s blood although the blood of the mother was infectious.  This was a very strong indication that the rods were the cause; the maternal and foetal blood are separated by a membrane through which fluids and substances in solution pass; but insoluble substances, even when very minutely subdivided, do not pass the membrane.  If the cause were a poison in solution, the foetal blood would have been as toxic as the maternal.  The blood of infected cattle was filtered through filters made of unbaked porcelain and having very fine pores which allowed only the blood fluid to pass, holding back both the blood corpuscles and the rods, and such filtered blood was found to be innocuous.  It was further shown that the rods increased enormously in number in the infected animal, for the blood contained them in great numbers when but a fraction of a drop was used for inoculation.  Attempts were also made with a greater or less degree of success to grow the rod shaped organisms or bacilli in various fluids, and the characteristic disease was produced by inoculating animals with these cultures; but it remained for Koch, 1878, who was at that time an obscure young country physician, to show the life history of the organism and to clear up the obscurity of the disease.  Up to that time, although it had been shown that the rods or bacilli contained in the blood were living organisms and the cause of the disease, this did not explain the mode of infection; how the organisms contained in the blood passed to another animal, why the disease occurred on certain farms and the adjoining farms, particularly if they lay higher, were free.  Koch showed that in the cultures the organisms grew out into long interlacing threads, and that in these threads spores which were very difficult to destroy developed at intervals; that the organisms grew easily in bouillon, in milk, in blood, and even in an infusion of hay made by soaking this in water.  This explained, what had been an enigma before, how the fields became sources of infection.  The infection did not spread from animal to animal by contact, but infection took place from eating grass