Vaccines for HIV, malaria, and Zika still evade us. REUTERS/Karoly Arvai One of the best defenses we have against viral infections is the vaccine.

Vaccines prepare the body's immune system to fight an incoming infection; they've been credited with the widespread eradication of smallpox and the near-eradication of polio.

But for all the conditions vaccines have succeeded in eradicating, there are still many diseases for which no vaccine exists.

Researchers are finding ways to use the immune-system-triggering effects of vaccines to tackle unexpected diseases, such as cancer and drug addiction.

To get approved, vaccines need to show that they're both safe and effective at preventing diseases or — if they're used therapeutically — at activating the immune system to go after existing diseases. That process can take years or even decades.

Here are nine vaccines currently in development that could dramatically change how humans live.