A viral strain may infect a host, proliferate rapidly, become controlled by immune reactions, and eventually be eliminated from the body. The virulence, or the magnitude of harm to the host due to infection, depends on the abundance and duration of the viral strain in the body, and the importance of the damaged tissue of the host. In this study, we investigated how the cumulative viral load (time-integral of the number of infected cells) depends on various factors, such as the viral growth rate, the effectiveness of immune cells to kill infected cells, speed of immune activation, formation of memory cells, and longevity of immune cells. In addition, viruses may produce a mutant with different antigen types, escape the immune reaction targeting the original type, and inflate virulence. We succeeded in deriving simple and explicit formulas of the virulence in four different parameter regions. We found that, in some parameter region, enhancing memory cells is very effective in suppressing virulence, but direct activation of immune effector cells is not effective; while the opposite is the case in other regions. The result could be important in designing drug suppressing virus effectively. Additionally, we discussed the reported correlation between virulence and molecular evolution rate.

病毒株可能感染宿主，迅速增殖，受免疫反应控制，并最终从体内清除。毒力或感染对宿主造成的伤害程度取决于病毒株在体内的丰度​​和持续时间，以及宿主受损组织的重要性。在这项研究中，我们研究了累积病毒载量（受感染细胞数量的时间积分）如何取决于各种因素，例如病毒生长速率、免疫细胞杀死受感染细胞的有效性、免疫激活速度、形成记忆细胞和免疫细胞的寿命。此外，病毒可能会产生具有不同抗原类型的突变体，逃避针对原始类型的免疫反应，并增强毒力。我们成功地推导出了四个不同参数区域的简单而明确的毒力公式。我们发现，在某些参数区域，增强记忆细胞对抑制毒力非常有效，而直接激活免疫效应细胞则无效；而其他地区则相反。该结果对于有效设计药物抑制病毒很重要。此外，我们讨论了毒力和分子进化率之间报告的相关性。