1. Some strains of Trypanosoma lewisi may, at times, produce rapidly fatal infections in a large percentage of the rats infected.
2. In such strains of Trypanosoma lewisi, a sufficient degree of pathogenicity may persist to warrant the designation of these strains as pathogenic.
3. The pathogenicity of a given strain of Trypanosoma lewisi is not constant, but is subject to marked and even sudden variations.