Medics working at refugee aid camps in the Balkans say they are seeing a spike in the number of migrants falling ill as freezing temperatures begin.
It has fallen to as low as -11C in the region.
Medical charities International Medical Corps and Medecins Sans Frontieres say most patients are suffering with respiratory problems such as bronchitis and flu.
There are also concerns about people refusing or not seeking treatment.
Migrants are offered medical assistance, warm clothes and food at the main refugee points at the Serbian border with Macedonia to the south, and Croatia to the north.
International Medical Corps runs a makeshift clinic at the train station in the tiny town of Sid, in northern Serbia.
"Last week, when temperatures were a bit less, we were seeing around 50 to 60 people a day," said Sanja Djurica, IMC team leader.
"This week, now that temperatures have fallen, it's more like 100 or so a day."
"Almost all of them are suffering with respiratory illnesses brought on by the cold."
“I met the Al-Maari family, who are making the journey as the snow falls thick and fast. They fled Syria three weeks ago, and have been on the road ever since. They are travelling with four children; the youngest is just two years old. His brother Mohammad, seven, has a fever and a chest infection.”
"We are on a journey of death," said Mohammad's uncle, Iyad Al-Maari.
"We can endure. But I am worried about the children - the cold, disease and hunger."
CREDIT: BBC
