In monitoring the incidence of intestinal parasites in children and employees of a nursery the authors examined 31 children with 8 (25.81%) and 16 employees with 3 (18.75%) positive results. The authors wanted to examine also the family members of 8 positive children and 3 positive employees but except from the cleaner's family, (Ascaris lumburicoides, Enterobius vermicularis and Entamoeba coli) nobody accepted the offer. All 8 members of a large family except for Patient 1 (a cleaner) and her grandson were without clinical and laboratory findings. They constitute 3 independent families who lived in 1st category flats. On August 31 there was an extensive sewage disposal failure in the ground floor flat of Family II and the flat was flooded by sewage. All family members worked solidarily on cleaning and also the members of Family IV who are friends of Family II. As shown by clinical symptoms of 'virosis', during the pre-patent period and after an outbreak within 73-78 days, laboratory findings of the family members demonstrated a severe family infection equal to a epidemic of intestinal parasitosis. Ascaris lumbricoides was diagnosed in 8 family members (61.54%) and Giardia intestinalis in 7 family members (53.85%) involved in cleaning. Enterobius vermicularis was found in 2 and Etamoeba coli in 1 family member. In monitored persons, in extreme hygienic conditions during the failure and later, a mass contraction arose on the basis of infection. The fact, that family epidemic arose subsequently, proved, in contrast to sporadic findings in children and adults, a 6.4 and 3.3 times higher incidence of Ascaris lumbricoides and a 5.6 and 8.6 times higher incidence of Giardia intestinalis. The authors discus the reasons of incidence and also preventive measures in population. (Tab. 3, Ref. 29.).