Answer the questions (4-5) according to the text below.
Here is the vaccination timeline from 1955 to 2016. Medicine science has developed quicldy in the last 100 years. Of course, many new diseases also occured and ended in the 20th century. In the 21st century, we see some viruses that have affected the people all over the world. Scientists are still studying on those diseases.
We don’t clearly understand from the chart that - - - -