Here they talk to AFP about what they have seen and how they are coping.
- 'I have to choose' -
Sara Chinchilla is a 32-year-old paediatrician who works at a hospital in Mostoles near Madrid. So huge is the influx of patients that staff must prioritise who will be taken into the intensive care unit where they have the best chance of survival -- namely the younger ones with no preexisting medical conditions.
"So if I've got five patients and only one bed, I have to chose who gets it," Chinchilla says.
"People are dying who could be saved but there's no space in intensive care."
Her hospital is also lacking material. Though it has recently received more masks, she says, what it needs most is respirators.
"Many more lives could be saved if we had respirators."
The staff too are close to breaking point because many of them have fallen sick -- "each day more are going down with it".
The hospital has been completely reorganised to take in the huge influx.
"There are no more departments for gynaecology, paediatrics or traumatology, right now the entire hospital is treating COVID-19," Chinchilla says.
"There are more and more patients and fewer doctors, it can't carry on like this."
In Spain, thousands of doctors, nurses and healthcare workers have been infected by the virus, officials say. At least three have died.
- 'Overwhelming loneliness' -
Regina Dalmau, 48, is a cardiologist at Madrid's La Paz hospital who has been treating coronavirus patients for weeks.
"When you leave the hospital, there's a sense of real sadness. These (patients) are all alone; when they die, they die alone and when you get home, you have to digest that, you have to cry. Nobody could have imagined this."
Dalmau has seen some "pretty harrowing situations", of patients living through some brutally short last moments.
"You call the family to come and say goodbye" but they can only come if they don't have symptoms and if they haven't been living with the patient over the last five days, she says.
"They might be there for 10 minutes but they can't go near (their loved one)... There is loneliness on both sides and it is overwhelming."
For Dalmau, the situation is "all-out war" with the worst "yet to come".
What Spain is experiencing now, with the death toll soaring past 4,000 and the number of cases rising above 56,000, is just the result of "people being infected two or three weeks ago", she says.
The authorities have "made a horrible mess of managing the crisis", she adds.
Just days before the March 14 lockdown, people were out en masse attending football matches, visiting Madrid's ARCO contemporary art fair or taking part in mass marches for International Women's Day on March 8.
"This virus has caused some serious blind spots," Dalmau says.
Sonia Pacho is a nurse who works at Galdacano hospital near the northern city of Bilbao, a hospital which hit the headlines last week after one of its nurses became the Spain's first healthcare worker to succumb to the deadly pandemic.
"It was a real blow, you feel very powerless," says this 48-year-old, who visits patients with mild symptoms across a wide geographical area that sometimes sees her driving more than 100 kilometres (60 miles).
- Masks don't last forever -
She carries out tests on people of all ages, and on each visit, she has to be kitted out in fully-protective gear: gloves, a mask, goggles, a gown and plastic covers for her shoes.
And in taking it off, she has to be scrupulously careful.
But for those working back at the hospital, such items aren't easy to get hold of, and not having protection "really limits what you can do".
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"I have colleagues who are reusing their masks over and over again," she sighs. "And masks don't last forever."
At the hospital, the atmosphere is tense, she says although the staff are demonstrating a lot of solidarity and willingness to help by stepping in to help out, changing shifts or doing extra days.
"If they called me in to help out, I would definitely go."
- Sidelined by sickness -
Irene Sanz, a paediatrician at a hospital in the northwestern city of Valladolid, has been at home with her two small children since testing positive for coronavirus on March 13.
"I had a fever of 39 degrees Celsius (102 degrees Fahrenheit) for several days, and the fever lasted for 10 days with a lot of muscular pain, exhaustion and a bit of a cough," she said. "I was really fed up."
Now she's recovered and is hoping that she will test negative for the virus next week.
"I want to go back to work because with all the staff falling sick, they really need more people," says Sanz, 35.
"But I'm also afraid about what I'm going to find when I get there."
By Alvaro Villalobos and Michaela Cancela-Kieffer