Aggressive behavior is observed among various species with diverse outcomes, such as establishing territory, dominance, discipline, obtaining prey avoiding predation, etc. (Wilson, 1975). When animals encounter other animals, they immediately decide whether to attack the enemy or escape from it. To judge the situation appropriately, the animal must assess information from the environment as well as the movement of the other animal. Therefore, aggression is a very complicated output behavior that occurs after the processing of much information. Aggressive behaviors of social animals, which live in kin groups, are more complex because they sometimes imply altruism. One example is the honeybee's aggressive behavior toward natural enemies. Honeybees are eusocial insects; female adults are differentiated into reproductive females and sterile females. The behaviors of sterile females are ecologically altruistic. Altruistic behavior is defined as behavior that provides advantages for others at the expense of one's own fitness (Wilson, 1975). For example, it is altruistic that workers who cannot bear their own daughters take care of the daughters produced by the queen. Among these labors, aggression is a typical altruistic behavior because the aggressors die as a result of the aggression to protect the colony, as mentioned in the next chapter. Altruistic aggression is also observed in other animal species (Wilson, 1975). Most of the mechanisms underlying these behaviors remain unknown, but their clarification will contribute to a better understanding of animal behaviors and sociality. We have been studying the molecular basis of aggressive behaviors in the honeybee. The advantages of studying aggressive behaviors in the honeybee are as follows: (1) It is easy to distinguish individuals based on their behaviors, because there is a division of labor in the sterile workers based on their age after eclosion. Therefore, the workers that exhibit aggressive behaviors are expected to have specific brain functions to express the programmed aggressive behaviors. (2) The brain structure of the honeybee has been extensively analyzed compared to other social insects (Ehmer and Gronenberg, 2004) and many experimental methods, such as molecular biology, electrophysiology, neurohistology, etc., can be applied to study the honeybee brain (Giurfa, 2003; Kunieda and Kubo, 2004; Menzel, 2001). (3) Honeybee aggression is often aimed toward human beings, resulting in stinging incidents. The Africanized honeybee in particular, a European honeybee and African honeybee hybrid, is highly aggressive and is called the killer bee because its sting can kill humans (Guzman-Novoa and Page, 1994b). These bees broadly inhabit North and South America, and stinging is a serious public health issue there. Therefore, analysis of the neural and molecular mechanisms underlying honeybee aggression is expected to contribute to public health, agriculture, and apiculture. For these reasons, we screened the genes preferentially expressed in the brains of aggressive workers. These experiments led to the discovery of a novel insect picorna-like virus, termed Kakugo virus, in the brains of worker honeybees that exhibited aggressive behaviors against hornets (Fujiyuki et al., 2004). It is possible that viral infection in the brain has a role in regulating the aggressive behaviors of the honeybee. In this article, we describe the findings on Kakugo virus, and discuss the possible relationship between viral infection and aggressive honeybee behavior. We also discuss other animal viruses that are thought to be related to the aggressive behaviors of the host.