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Virulence is a key component of parasite fitness. Its expression and selective value may not only depend on the features of the parasite's life cycle, but also on host genotype or environmental conditions. Using the freshwater ciliate Paramecium caudatum and its endonuclear bacterial parasite Holospora undulata, we measured variation in virulence (reduction in host division and survival), parasite load and fidelity of vertical transmission for (i) different stages of infection (associated with different opportunities for vertical and horizontal transmission), (ii) different host clones, and (iii) two food conditions. Later stages of infection dedicated to horizontal transmission were more virulent than earlier stages which rely on vertical transmission only. Besides, investment in horizontal transmission decreased the efficacy of vertical transmission, indicating a tradeoff between the two pathways. This may explain the phenotypic plasticity of transmission mode of this parasite. To some extent, virulence, parasite load and transmission fidelity varied with host clone identity and food treatment (higher virulence at low food). These results suggest that virulence is not a constant property of the parasite, and that a single (and simple) relationship between virulence and transmission does not exist.