Figure schematic life-cycle (source: helminth control in school age children)
Soil-transmitted helminths live in the intestine of infected individuals where they produce thousands of eggs each day that are passed in the faeces. Where the environmental conditions are favourable, the eggs develop into infective stages.
Humans become infected when ingesting infected eggs (Ascaris lumbricoides and Trichuris trichiura) or larvae (Ancylostoma duodenale) in contaminated food (e.g. vegetables that are not carefully cooked, washed or peeled), hands or utensils or through penetration of the skin by infective hookworm larvae in contaminated soil (Necator americanus and Ancylostoma duodenale).
There is no direct person-to-person transmission or infection from fresh faeces because eggs passed in faeces need about 3 weeks in the soil before they become infective.