Does gingivitis ALWAYS precede periodontitis?
Or can periodontitis just suddenly appear without having gingivitis or any warning?
I'm very afraid of getting periodontitis because it is IRREVERSIBLE. My cousin has periodontitis and I'm always afraid that her gum-disease causing bacteria will eventually infect me since I know that these bacteria can be transmitted through saliva when sharing drinks. 
Comments
YES, gingivitis must be present first and from there it goes to periodontitis.
Not always. Some cases of periodontitis are just onslaught by poor dental Hygiene. The best way to avoid any complications with the gum tissue is to floss regularly, and use a good toothpaste that has an anti-gingivitis component, crest pro-health clinical works good.
gingivitis precedes periodontitis 100% of the time.
You know what's fun? You deliberately leave a whole brick of unused firecrackers and a book of matches at the playground the day after the 4th of July, where you know the 5th Graders hang out... Maybe throw in a couple of M-80's and make bets on how many fingers or mailboxes get blown off...
http://www.perio.org/consumer/2a.html <-- thats a good website about gum disease.
My advice is brush twice a day with soft toothbrush, floss and eat healthy and get your teeth clean by a dentist every 6 months,
I agree with the doc, but I wouldn't use pro-health, I find it's too abrasive and therefore too hard on your enamel.