The minimum number of propagules of a parasite required for infection of an organism to take place under favourable conditions (E. Gaiimann, Principles of plant infection, 1950, London, Crosby Lockwood). The term implies that some minimum number of propagules must be present before infection occurs; however, some parasites have an NTI of 1, e.g. Botrytis fabae and Phytophthora infestans. In the case of soil-borne parasites calculations of the numerical threshold of infection are frequently based on the number of diseased plants, which may give a large underestimate of the numbers infected. Also, the number of propagules required for infection in soil depends on the statistical probability of contact being made between the infective unit and the susceptible host surface, i.e. the number required to ensure contact may greatly exceed the number that need to be present at the point of contact in order to cause infection.