How does Louis Pasteur’s 1880s method of injecting weakened viruses help fight viral diseases? A) The immune system develops antibodies in response to the weakened viruses. B) The weakened viruses attach to unaffected viruses in the host and interrupt the viral reproductive cycle. C) Weakened viruses are unable to enter the host organism. D) The rate of genetic mutation in the host is decreased due to the introduction of weakened viruses.

What this question is really asking

Pasteur’s “weakened virus” method describes vaccination with an attenuated pathogen. The key idea is not that the weakened virus fights other viruses directly, but that it trains your immune system.

Why injecting a weakened virus works

When an attenuated virus enters the body, it still has viral antigens (surface proteins) that the immune system can recognize. This causes:

• Activation of immune cells
• Production of specific antibodies that bind to that virus
• Formation of memory B cells and memory T cells

Later, if the person is exposed to the real (fully virulent) virus, the immune system responds much faster and stronger, often stopping the infection before serious illness.

Checking each option

• A is correct: antibodies (and immune memory) develop in response to the weakened virus.
• B is incorrect: viruses do not typically “attach to other viruses” to stop their replication in a host.
• C is incorrect: a weakened virus can still enter host cells; it is just less able to cause disease.
• D is incorrect: vaccination does not work by reducing the host’s genetic mutation rate.

Quick takeaway

Vaccines protect because they expose the immune system to antigens safely, leading to antibodies and immune memory.