18. Most vaccines are given to children and protect them for life. However, doctors recommend that people get a flu vaccine every year. Knowing that the flu is caused by a virus, why is this vaccine recommended much more frequently than others?

Viruses reproduce so quickly that the flu vaccine would not prevent illness from the virus's offspring.

The flu vaccine does not kill viruses; it simply causes the virus to lay dormant for a short period of time.

Viruses constantly change, creating variants that may not be affected by the previous year's vaccinations.

Viruses can adapt to resist bacterial antibiotics, causing humans to take more medicine than previously needed.