Nichiren [and the Lotus Sutra] teach,
"Thus you should understand that so long as a practitioner of the Lotus Sutra remains unwavering in faith, free of all false alliances, entrusting himself wholeheartedly to the Lotus Sutra and practicing in accordance with the Buddha’s golden words, he will without fail be able to prevent disaster and prolong his life in this present existence, to say nothing of in the life to come. Splendid recompense will be his, and he will fulfill his great vow to broadly proclaim and propagate the Lotus Sutra." -- Nichiren
"This good excellent medicine now I leave here. You should take and swallow it. Do not worry that you will not be cured." -- Lotus Sutra Chapter 16
"This Sutra then is the Excellent medicine for the sick people of Jambudvipa. If there shall be sick people who are able to hear this Sutra, the sickness shall at once dissipate and they shall not die before they grow old." -- Lotus Sutra Chapter 23
This is especially true in the case of a young person who embraces the Sutra according to Nichiren.
Since one will definitely overcome their illness with a correct faith, for example, Jerry Marcheso overcame a large malignant brain tumor and Greg Romero, a massive heart attack and stroke because of their correct faith. But Shin Yatomi was unable to overcome his illness precisely because his faith was warped ["I will definitely overcome my illness since I am a disciple of Sensei." -- Shin Yatomi]