Gemara

(R. Yitzchak brei d'Rav Yehudah): One should pray that he not get sick, for one who is sick needs merits to be cured.

(Mar Ukva): He learns from "Ki Yipol ha'Nofel Mimenu" - if one falls, proof is required from him (that he merits to be saved).

(Beraisa): One who has a slight headache should view himself as if he has a noose around his neck (and is being judged). One who is bedridden due to illness should view himself as if he was being judged in Gardom (where they try the most serious offenders, almost all are killed);

One in Gardom is not saved unless he has great advocates.

Berachos 54b (Rav Yehudah): Four categories of people must thank Hash-m -- one who traveled through a wilderness, a sick person who recovered...

55b: When Rava fell sick, the first day he would not reveal it, lest his Mazal turn bad. After this, he asked his servant to announce it, so those who love him will request mercy for him.

Rishonim

Rosh (Berachos 9:3): Rabbeinu Yosef says that only a Choleh who was bedridden says ha'Gomel, but not one who had a pain in his head or stomach. The Aruch connotes that one blesses even for these.

Maharam Mintz (14): A clever 12-year old was saved from a mortal sickness. He cannot say the Berachah that Chachamim enacted, for he cannot say "Gomel Chasadim Tovim la'Chayavim", for he is not yet liable. Or, perhaps he is liable due to Aveiros in a previous Gilgul (incarnation), even though he is not liable for anything he did since he was born. Also, he can bless in any case. If evil occurrences came to him due to Gilgul, he may bless "Gomel Chasadim la'Chayavim." If this is his first Gilgul, we must say that he is being punished due to his father. We say that children die due to vows of the parents. Even though his father was Chayav, he did not die.

Question: If so, a father should bless when his son recovered!

Answer (Maharam Mintz): The father is unsure whether the sickness is due to himself or his son's previous Gilgul. We do not find a Berachah enacted due to Safek. The son may bless, for in any case due to Hash-m's Chesed he did not die. Also, the father has a Safek whether it was proper for the son to die, to save him from great Aveiros in the future, like we say about Ben Sorer u'Moreh.

Maharam Mintz: Perhaps the son cannot bless. If the sickness was due to his father, he cannot say 'Chayavim' about his father. Chachamim did not enact for one to say so about his father. One may not disgrace his father. If he sees his father transgress, he may not directly say so, even in private. All the more so he may not publicly call his father Chayav, even though it is a Safek and perhaps he refers to himself due to a Gilgul. Letter of the law, the son should bless Gomel Chesed without mentioning 'Chayavim'. However, we do not deviate from the text that Chachamim enacted for Berachos. I conclude that the son should not bless.

Poskim

Shulchan Aruch (OC 219:1): A Choleh who recovered must thank Hash-m.

Mishnah Berurah (2): This is when he now walks like a healthy person.

Kaf ha'Chayim (7): Therefore, one who gave birth does not bless until after seven days, for then she is totally healed.

Kaf ha'Chayim (10): If one choked on a bone and was saved, Divrei Chachamim was unsure whether he blesses. Therefore, he blesses without Shem and Malchus. The same applies to one who fell from a ladder and his limbs hurt, but he was not bedridden.

Birkei Yosef (1): Maharam Mintz said that the son should not bless. Lachmei Todah (5) disagreed, and said that a minor may bless. This is our custom.

Mishnah Berurah (3): The Acharonim say that a minor is not obligated to thank (even due to Chinuch, i.e. training in Mitzvos).

Kaf ha'Chayim (1): In Eretz Yisrael we never heard a minor bless. A minor should not bless where this is not the custom, for we are lenient about Safek Berachos.

Shulchan Aruch (8): One must bless for any sickness, even if it is not dangerous or internal. Anyone who was bedridden and recovered must thank, for this is like one who was tried for a capital case. We do not distinguish between whether or not the ache comes regularly.

Kaf ha'Chayim (44): Even if he knows that the sickness will return, he blesses each time he is healed from it.

Kaf ha'Chayim (48): If one drank poison and the doctors were able to cure him, he blesses.

Magen Avraham (7): Even though one is regularly healed from it, miracles do not happen every day

Rema: Some say that one blesses only on a dangerous sickness, e.g. an internal wound (the Tur citing the Ra'avad and Rabbeinu Yosef. This is the custom in Ashkenaz.)

Bach (4): There is no hint that the Rambam obligates blessing for every sickness. The Tur cites the Ramban, who learns from the Yerushalmi that every Choleh must bless. All Cholim are b'Chezkas Sakanah', i.e. those who have dangerous sicknesses.

Magen Avraham (8): The Bach rules like the first opinion. Some have this custom.

Taz (5): The same applies to a sickness that made him bedridden for more than three days. Then, he needs others to request mercy for him, like Rava did.

Eliyahu Rabah (13): The Bach's opinion is primary. Why did the Rema write oppositely in the name of Rabbeinu Yosef? Rabbeinu Yosef holds like the first opinion! Only regarding aches of the head or eyes, the Tur says that also the Ra'avad says that one does not bless. Rabbeinu Yosef agrees about this. Only the Ra'avad requires a dangerous sickness. Rabbeinu Yosef holds that one blesses whenever he was bedridden. The Rosh, R. Yerucham and Avudraham did not bring the Ra'avad at all, for he is a lone opinion. Maharil writes that one who had a fever blesses. One who walked under a leaning wall does not bless because it is not considered such a danger. One passes quickly under it.

Bi'ur Halachah (DH Kegon): Maharil did not specify for which fevers one blesses. I say that it depends. If he holds like the Mechaber, it is even for a (simple) fever. If he holds like the Rema, it is only for fever with shivering. The Taz says that one blesses only if he was bedridden for more than three days. This Shi'ur appears in YD 335. Strangers visit a Choleh only after three days. The Gra says that after this, he is called a Shechiv Mera. This is not relevant here. The Rema says that we bless only for internal wounds or similar matters. The Shi'ur of three days was not mentioned regarding Chilul Shabbos. That depends only on what doctors consider dangerous, e.g. internal wounds, or fevers. The same applies here. Three days could be relevant according to the Mechaber. Then, he needs mercy. However, the Mechaber connotes that anyone bedridden blesses. Perhaps the Taz makes his own mediating opinion. This requires investigation.

Gra (DH v'Yesh): This argument depends on the argument in Sa'if 7 (about whether one blesses for journeys through settled areas, which are not so dangerous).

Mishnah Berurah (22): When one is brought to trial, we do not know what the final verdict will be. Similarly, even a sickness that is not initially dangerous, sometimes it worsens and becomes dangerous.

Mishnah Berurah (28): The same applies to any dangerous sickness, e.g. a fever with shivering of the entire body. Some rule like the Mechaber. Magen Giborim says that whenever the entire body is sick in a way that we may tell a Nochri to be Mechalel Shabbos for him, one blesses ha'Gomel. The Chayei Adam adds that this is only if he was bedridden for three days, or if it is dangerous.

Halichos Shlomo (ha'Gaon R. S. Z. Auerbach, Ztz"l, 23:8): If one has a doubt whether he must bless ha'Gomel (for a flight or recovery from illness), he should intend to be Yotzei in Birkas ha'Shachar in 'ha'Gomel Chasadim...', and say it in front of 10, including two Rabanan, if possible.

Teshuvos v'Hanhagos (2:136, citing the Steipler): In any case it is proper that he sing Shirah to Hash-m, e.g. Mizmor l'Sodah or Nishmas.