Human disease may result from contaminated soil, due to unsanitary practices for disposal of excreta, improper or inadequate sewage treatment, or unfavourable climatic conditions. Basically, the types of diseases emanating from soil contamination can be divided into three main categories. Man-soil-man diseases result from contamination of soil by human excreta from which disease is contracted by either direct skin contact or consumption of food grown in such soil; examples include enteric bacteria and protozoa, and parasitic worms (helminths). Animal-soil-man diseases result from contact with soil previously contaminated with excreta of animal carriers, cadavers, and any part of infected animal bodies. Among this latter category can be found anthrax, leptospirosis, and Q fever. The third category of disease is a result of fertile climatic conditions inducing the proliferation of a pathogen from microorganisms growing in the soil. The mycoses, tetanus and botulism join this subdivision. Many of the diseases result from inadequate pretreatment of soil reused as fertilizer, or waste water reclaimed for irrigation purposes. Conventional sewage treatment processes cannot remove all the pathogenic organisms, although success of removal generally parallels removal rates for coliform organisms. For practical purposes, it cannot be assumed that even a well-run biological sewage treatment plant can consistently remove more than 90% of the pathogens from the sewage.
As an indication of the widespread nature of the problem, it has been estimated that about one third of the world's population is infected by hookworm, while one out of every four people in the world may be infected with [Ascaris lunbricoides].