title: Creating new variable in R: a proportion of 2 existing variables
body: <p>I would like to analyse the proportion of infected bees(DWV/TOTAL) in function of time (DAY_SINCE_TREATMENT), but how do I create a new variable: the proportion of infected bees (DWV/TOTAL)? 
  The dataset looks like this:</p>
  
  <pre><code>      COLONY DAY_SINCE_TREATMENT CTRL DWV TOTAL  
      1      A                  11    0   1     1  
      2      A                  13    4   3     7  
      3      A                  15   17   8    25  
      4      A                  17    3   0     3  
      5      A                  18    7   1     8  
      6      A                  19    6   1     7  
  </code></pre>