As a young microbiologist working with colleagues on the first Ebola outbreak in 1976, we had no clue at first how the virus was transmitted. The following account describes the time in the Democratic Republic of Congo when we began to understand how the virus was spreading.

"Night by night, as we jotted down data and sketched out a picture from our interviews and notes, it appeared that although people were still dying (and dying horribly), the peak number of new infections around the Yambuku mission might be, at least provisionally, behind us.

This was a huge relief. But another conclusion also began to take shape, and it was a great deal more uncomfortable to deal with. Two elements linked almost every victim of the mystery epidemic. One factor was funerals: many of the dead had been present at the funeral of a sick person or had close contact with someone who had. The other factor was a presence at the Yambuku Mission Hospital. Just about every early victim of the virus had attended the outpatient clinic a few days before falling ill.

We developed near-certitude about the mode of transmission one evening, when Joel [Breman] and I were drawing curves showing the number of cases by location, age and gender. (Working with Joel was a real education, like a terrific crash course in epidemiology.) It seemed likely by this point that aerosol contact was not enough to transmit the disease. But particularly in the 18 to 25 age group, at least twice as many women had died as men. We knew that there was something fishy about the hospital, and about funerals, but this was the real clue. What's different in men and women at that age?

Being a bunch of men, it took us a little time to figure out the answer. Women get pregnant. And indeed, almost all of the women who had died had been pregnant, particularly in that age group, and they had attended the antenatal clinic at the Yambuku mission.

Masamba [Matondo] and [Jean-Francois] Ruppol were the first to figure out the picture. Vitamin shots. They were usually completely pointless, but many African villagers considered them vital: to them the act of injection with a syringe was emblematic of Western medicine. Thus there were two words for Western medicine in the region. Anything ingested orally was 'aspirin', and it was hopelessly weak. An injection was 'dawa', proper medicine – something strong and effective.

We needed to take another tour of the Yambuku hospital. Knowing what we now did, the empty rooms and bare metal bed frames of the mission hospital seemed more disturbing – grim killers of the joyful young mothers who had come there to be cared for but left with a lethal disease. When we reached the stock room, we hunted through the large multi-dose jars of antibiotics and other medications. Their rubber bungs had been perforated multiple times by syringes. In some cases the bung had been removed and was stuck down with a simple bandage. Nearby were a few large glass syringes, five or six.

We politely interviewed the nuns. Sister Genoveva told us quite freely that the few glass syringes were reused for every patient every morning, she told us, they were quickly (and far too summarily) boiled, like the obstetric instruments employed in the maternity room. Then all day long they were employed and re-employed; they were simply rinsed out with sterile water.

She confirmed that the nuns dosed all the pregnant women in their care with injections of vitamin B and calcium gluconate. Calcium gluconate is a salt of calcium and gluconic acid; it has basically no medical value in pregnancy, but it delivers a shot of energy, and this temporary 'high' made it very popular among patients.

In other words, the nurses were systematically injecting a useless product to every woman in antenatal care, as well as to many of the other patients who came to them for help. To do so, they used unsterilised syringes that freely passed on infection. Thus, almost certainly, they had unwittingly killed large numbers of people. It looked as though the only obstacle to the epidemic had been the natural intelligence of the villagers, who saw that many of the sick came from the hospital, and thus fled it; who knew to set up at least some barriers to travel, thus creating a semblance of quarantine.

The nuns were totally committed women. They were brave. They faced an incredibly difficult environment and they dealt with it as best they could. They meant well. We had shared their table and their lives for what seemed like far longer than four days, and every evening, as they sipped their little tots of vermouth, they had told us about the villages of their childhoods. Every evening the discussion had ended up circling around and around the same subject – the epidemic: who had fallen ill first, when and how it had happened; the dread of infection; the horrible deaths of patients and colleagues.

They had been trying to map out the frightening terrain until, I suppose, it would seem more manageable, less horrific. It was a narrative in which they had felt like heroes of a sort, and certainly martyrs.

Now it appeared that they were in some sense villains as well. It was very hard to formulate the words that would inform the sisters that the virus had in all likelihood been amplified and spread by their own practices and lack of proper training. In the end I think we were far too polite about it: I'm not certain at all that it really sank in when we told them our preliminary conclusions.

Adapted from the book No Time to Lose: A Life in Pursuit of Deadly Viruses by Peter Piot, with permission of the author and publisher, W W Norton & Co