What type of substance is described as 'virulent'?

The term 'virulent' is used primarily to describe substances, particularly toxins or pathogens, that are extremely harmful or poisonous. In the context of biology and medicine, it often refers to highly infectious or virulent strains of bacteria or viruses that can cause severe disease or even death. The essence of virulence lies in its capacity to cause significant harm or injury, thus making it synonymous with being extremely poisonous or injurious.

The other options describe substances that imply safety or a lack of harmful effects, which stands in stark contrast to the definition of 'virulent.' A completely harmless substance would imply no potential to cause illness, while moderately safe suggests a level of safety that does not resonate with the inherent danger of virulent agents. Similarly, substances that require careful handling may still be relatively safe as long as precautions are taken, but they do not encapsulate the acute danger associated with something that is virulent.