In the centuries that followed, accusations and discussions of biological warfare were common. Well-documented accounts of incidents involving both threats and acts of deliberate infection are very rare, but may have occurred more frequently than scholars have previously acknowledged. Many of the instances likely went unreported, and it is possible that documents relating to such acts were deliberately destroyed, or sanitized. By the middle of the 18th century, colonists had the knowledge and technology to attempt biological warfare with the smallpox virus. They well understood the concept of quarantine, and that contact with the sick could infect the healthy with smallpox, and those who survived the illness would not be infected again. Whether the threats were carried out, or how effective individual attempts were, is uncertain.