This one doesn't involve any real math all too much, but I'm still getting confused on it, help? Thanks!
The rate R that a certain disease spreads is proportional to the number of infected individuals and is also proportional to the number of uninfected individuals. The total population is P and the number of infected individuals is D. Express the rate that the disease spreads in terms of this information. Use C as your proportionality constant.
(EDIT: I can't believe I misspelled "fairly" in the title, sorry!)