The factors mainly determining the losses caused by animal diseases, and the manner in which the economic effects are felt by the various groups of those concerned, are considered with reference to each form of disease. The losses on farm level due to those diseases among dairy cattle, which are common in most areas but vary in degree on each farm are discussed in greater detail. The losses are quantitated in part after differentiation of three components, namely prior to disposal (less efficient production and cost of veterinary treatment), on disposal (reduced slaughter value and idle production factors), and missed future income due to forced replacement. Moreover, the decision to replace animals is considered, also with reference to cows with disease. Finally, the question of which extension of veterinary knowledge is desirable to promote economic research, is discussed.