In immunosuppressed individuals, such as a calf with BVD, the infective dose required to cause illness is generally smaller.

• A. True
• B. False
• C. Not always
• D. No relation

Answer: (A)

Think about how the amount of pathogen needed to cause illness depends on how well the host defends itself. When the immune system is strong, it takes a larger dose to establish infection and push it toward disease. When someone or an animal is immunosuppressed, those defenses are weakened, so even a small number of pathogens can slip past barriers, establish infection, and sometimes progress to illness.

BVD can cause immunosuppression by damaging immune tissues and reducing white blood cells, which lowers the animal’s ability to mount an effective response. That means a calf with BVD is more likely to develop illness from a smaller infectious dose than a healthy calf.

So the statement is true: in immunosuppressed individuals, the infective dose required to cause illness is generally smaller.