Some viruses can enter cells by first attaching to the cell membrane. The flu virus targets and attaches to the cells of the nose and mouth. The hepatitis virus targets only specific cells of the liver.

Many organizations release indexes used to measure the development of the world's countries. as we learned in this lesson, these indexes measure many factors, from life expectancy to happiness. in your opinion, what are the three most important factors we can use to determine how developed a country might be? explain your answers in a few sentences.