conditional probability

#1
For a combined ELISA-Western blot blood test for HIV positive status, the sensitivity P(positive test |HIV) is about 0.999 and the specificity P(negative Test | not HIV) is about 0.9999.

Consider a high risk group in which 10% are truly HIV positive. Construct a tree diagram to summarize this diagnostic test

I don't even know how to start. I've watched so many youtube videos and I'm still lost
 
#2
For a combined ELISA-Western blot blood test for HIV positive status, the sensitivity P(positive test |HIV) is about 0.999 and the specificity P(negative Test | not HIV) is about 0.9999.
If you know this information, then don't you also know P(negative test|HIV) and P(positive test|not HIV)?