William Campbell, Satoshi Omura and Youyou Tu have won the 2015 Nobel Prize in medicine.
Irish-born Campbell and Omura, a Japanese citizen, were cited for discoveries concerning a novel therapy against infections caused by roundworm parasites.
Tu of China was rewarded for discoveries concerning a therapy against malaria.
"These two discoveries have provided humankind with powerful new means to combat these debilitating diseases that affect hundreds of millions of people annually," the Nobel judges in Stockholm said in a statement.
The 2015 Nobel season continues on Tuesday (local time) with the announcement of the physics prize, and the chemistry prize on Wednesday.
On Friday, all eyes and ears will turn to Oslo for the Nobel Peace Prize announcement.
This year the prizes are worth eight million Swedish kronor (NZ$1.4 million). In line with tradition, the awards are presented to recipients on December 10, the anniversary of Nobel's death.