• You're misusing the { } brackets.
  • Variables have to be set inside of events.
  • msg $chan /me does not do a /me, you should use /describe $chan to do that.
  • Timers put out a yucky message when you start them, so you'll more than likely want to use /.timer


Code:
on *:TEXT:!cast aids *:#:{
  var %mod = [M]tree
  if ($nick == %mod) && ($3 ison $chan) { 
    describe $chan begins to cast AIDS.
    if ($rand(1,4) == 1) {
      msg $chan AIDS failed.
      return
    }
    .timer1 1 5 describe $chan has sucesfully casted AIDS on $3!
    .timer2 1 10 msg $chan $3 has 5 minutes till AIDS will take affect. 
    .timer3 1 70 msg $chan $3 has 4 minutes remaining till consumed by the AIDS effect.
    .timer4 1 130 msg $chan $3 has only 3 minutes remaining.
    .timer5 1 190 msg $chan $3 has only 2 minutes left! GASP!
    .timer6 1 250 msg $chan $3 has only 1 minute left! Please apply the !cure soon.
    .timer7 1 310 kick $chan $3 $(|,) msg $chan DIED OF AIDS 
  }
}

on *:TEXT:!cure:#:{
  msg $chan Cures can be purchased using AIDSbot credits. Currently the credit system is down so if you are currently suffering from AIDS there is no hope for you.
}