Prevention is simple:
Brush your teeth in the morning, after meals and snacks, and before bed using fluoride toothpaste.
Try an electric toothbrush that has a rotating and oscillating (back-and-forth) action. This type of toothbrush is better at removing plaque.
Use an antiseptic mouthwash, such as Listerine, or a fluoride or antiplaque mouthwash.
Floss once a day.
Use disclosing tablets periodically so you can see if you are brushing effectively.
See your dentist regularly to have your teeth cleaned and checked.

No, you won’t always have it as it is treatable..especially in the early phases. If you ignore it, it will progress and tissue damage and lead to permanent tooth loss.

Long answer not likely, Periodontal desease is a progression like hair loss. Gingivitis (swollen red gums) is a sign of active periodontal disease. Techniquely no one has periodontal disease until you can quantify any amount of boneloss anywhere in the mouth. Once you loose the bone it never comes back. Therefore once you have periodontal disease you always have periodontal disease. Gingivitis is a state of active periodontal disease. It is possible to have periodontal disease but lack gingivitis.

Then you are said to have controlled periodontal disease.

Periodontal disease is further classified into mild, moderate, and severe.

Most people have some type of mild periodontal disease by mid life, 1/2 mm here and there. It’s not a big deal, unless you are at the periodontist or under oath.

Weather or not it is contageous is a matter of semantics. If you mean can you catch the bacteria that cuase periodontal disease sure. Are you likely to keep them long given your hygiene is even reasonably good, NO! So technically periodontal bacteria is contageous. But realistically NO! it is not
Good luck