The concentration of pathogen needed to kill 50% of an infected group of test animals is the

LD50 (median lethal dose)

Explanation:

Whenever one needs to quantify the virulence of a pathogen then this is done by using a set of laboratory animals.LD50 (median lethal dose) is an important indicator of virulence. LD50 is defined as the number of pathogens that are required to kill 50% of the total infected animals.For the calculation of LD50, the animals are infected with a known number of pathogens and then a graph is plotted for the percentage of animals killed against the concentration of the pathogen used for infection. The point at which the graph indicates the death of 50% animals, the corresponding concentration of inoculation of a pathogen is taken as the LD50 of that particular pathogen. 

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