<span>There wasn't many vaccinations back in the day and medicine slowly developed through years passing by. Also eras such as the Black plague and others really helped develop medicines and vaccinations moving forward for people to use.
Hope this helps.</span><span />
The answer to your question is the letter B
Hope this helps you. but for this sort of stuff I suggest you look on the internet first
A organism is an individual animal, plant, or single celled life form