Eating out, poor hand hygiene and erratic weather may be among causes, say medics
Muriel, who is on loan from Sevilla, didn't celebrate the first but couldn't be faulted for expressing his joy after his spectacular second. Fabio Quagliarella also scored twice as the veteran forward continued his remarkable goalscoring run.
Sampdoria moved a point below the final Europa League qualifying berth and two points below Lazio, which visits second-placed Napoli later. Fiorentina was three points further back.
Muriel, who also played for Udinese and Lecce in Italy, joined Fiorentina at the beginning of the month.
The Colombia international had spent 1 1/2 seasons at Sampdoria before Sevilla broke its club record to sign the forward in 2017.
He scored the opener against his former club in the 34th minute, holding off two defenders before firing across into the bottom right corner.
His second was more spectacular as he turned Joachim Anderson just inside the Fiorentina half and sprinted downfield before dispatching a diagonal shot.
That restored Fiorentina's lead after the home side had Edimilson Fernandes sent off following two yellow cards and seen Gaston Ramirez equalise just before halftime with a stunning free kick.
Quagliarella thought he had won it for Samp with an 81st-minute penalty - after Vitor Hugo's handball - and another goal four minutes later.
It was his 12th goal in his past 10 league matches - impressive for a player who turns 36 on Jan. 31.
However, there was another twist to come as German Pezzella flung himself at Federico Chiesa's deflected cross to equalize with the last action of the game.
Duvan Zapata became the first Atalanta player to score four goals in a single Serie A match since 1952.
Gianluca Mancini headed in the opener before Zapata ran riot in Atalanta's 5-0 win over relegation-threatened Frosinone.
The Colombia forward has 14 goals in Atalanta's last eight matches in all competitions.
Bologna also remained in the relegation zone after it drew 1-1 at Spal, which was four points above the bottom three.
Empoli, two points from the drop, played Cagliari later Sunday.
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