I disagree. Weight training can be very valuable.
However, nearly every weightlifter I've ever met overtrains. If you are working out more than twice a week, you're overtraining. As you get bigger, you need to train LESS. Your muscles grow, but the body's ability to recover (which is when muscles develop!) does NOT grow, so as you get bigger...you need to rest more...which means working out less.
Serious weightlifters should also be sleeping 10 hours a day, which few do.
If you overtrain, don't sleep enough, don't eat right, then you'll get sick. If you train sensibly, sleep well, eat right, and are otherwise healthy, you will have no problems.
Additionally, most weightlifters jerk heavy weights because they want to look good at the gym (I'm not implying this is David). I highly recommend High-Intensity Training (aka Arthur Jones)...it's not a course or a book, but a method of training that emphasizes super-slow movements, absolute fatigue, and plenty of rest. Arthur Jones could give you just the barbell and work you until you threw up - there's no need to be jerking giant weights around.
Finally, remember that our bodies were built to do manual labor all day. Your body is the same model that Joshua or the Maccabees had, and they did not sit in front of a computer terminal all day.