The most riveting world chess championship ever by far was when Fischer won the title in 1972.
First, the rules were bent to accommodate him in the inter-zonals, because he had disappeared from the chess scene after having abandoned the 1967 inter-zonal tournament, which he was clearly winning, without any explanation, only saying "Leave me in peace, I have nothing to say", and the 1969 zonal championship for US (Zone 5, same as the US national championship) had been completed without him. His inclusion became necessary as he was clearly a leading candidate for the title judging by his recent play in the USSR vs the Rest of the world and other tournaments in which he had easily beaten the top Russian players.
Then, he won the last 7 games of the interzonal qualifier, without a single loss or draw (in fact he won 10 of the last 12, drawing 2; total games 23).
In the next stage, the 3-phase candidates knock outs, he beat Mark Taimnov 6-0 in the quarters; at the half way stage of their match, 3-0, Taimnov took a few days off on medical grounds, but it didn't help. He lost the next 3 games too. He was later quoted as saying, "I still have my music" (He was a professional pianist too).
When the dashing Larsen too fell sick after losing 4 straight games in the semis against Fischer, and also lost 6-0, there was talk of Fischer fever or Fischer fear. At this point, he had won his last 19 games consecutively at the highest level of chess.
His last opponent before the world title match against Spassky was the genial, but iron-willed former world champion, Tigran Petrosian, known for wearing out his opponents, playing for draw after draw, until the weary opponent was at the end of his endurance. For example, he had won his semi-finals match against Korchnoi by drawing the first 8 games, winning the 9th, and then drawing the last game.
Fischer narrowly won the first, then, finally, lost the second, his first loss after 20 consecutive wins in 20 consecutive games. Then, ominously for Fischer supporters, the next 3 games were drawn; this was Petrosian's speciality, and seemed to indicate that he was gaining the edge. There was a sense that Fischer was tiring, especially as he had actually accepted a grandmaster draw in the 4th game that lasted only 20 moves. This was not like Bobby Fischer at all. Perhaps Petrosian, with his tedious and unenterprising style of play, was the antidote for the Fischer virus.
The 6th game was a different matter, however, and Fischer was his old self again, going relentlessly for a win with a narrow advantage, and Petrosian, fighting grimly, finally resigned after 66 gruelling moves. He then lost the 7th too, in which Fischer twice rejected offers of a draw, having a very slight positional advantage. This loss appeared to be decisive as it seemed to break Petrosian who too now fell ill for 4 days, returning to the chess table to lose the next 2 games also, and the candidates' finals. The super-cautious Petrosian who had hardly ever in his career lost even 2 games in a row against top notch grandmasters was handed 4 consecutive defeats by Fischer.
Later it emerged that Fischer was actually not fully fit, having a severe cold, during games 3,4 and 5, but refused to take time off which might boost his opponent's confidence. He had calculated correctly in keeping up the unremitting pressure on the forty two year old Petrosian, who broke first under the strain.
Spassky, too, had to take time off for health reasons during the world championship finals, twice in fact, but Fischer couldn't quite steam-roll him as he had done the others, although Fischer emerged clearly as the better player. Almost as a matter of pride, he never asked for offs for health or any other reason.
Fischer fear was a genuine phenomenom and not a creation of the press or a publicity stunt.