Jessie Cave, who played Lavender Brown in the Harry Potter films, has said it was a "scary start" for her newborn baby after he tested positive for coronavirus.
The actress (33) welcomed Tenn in October after a "traumatic" delivery that left him in the neonatal unit.
She said the baby is out of hospital and is doing well, but her partner now has Covid and the stress has caused her breast milk to dry up.
"Nothing has gone to plan, which I guess everyone can relate to this year. It was a really scary start," Cave told ITV's Lorraine.
"He was in the neonatal unit after a really sudden, quick labour.
"He came out within 45 minutes, so he was just a bit like, 'What's going on?', and was taken to the neonatal unit. He's 11 weeks now.
"It's been such a strange period of time for everyone, but very scary."
Discussing how Tenn may have contracted the virus, Cave said: "All I know is it's absolutely the most contagious thing.
"I was completely symptomless, so it's just bizarre.
"And he was completely fine. There was absolutely nothing wrong.
"Then over a week, almost two weeks later, he just suddenly turned.
"Within a day of realising he was just kind of changing a bit, he wasn't feeding as much and his crying was different, it was like a different baby.
"I just went for it and I went into A&E. I think that's what everyone has to do.
"They have to be aware of a sudden quick change, because it could have been a completely different story if I hadn't gone in.
"You really do have to be so quick to go in and don't be scared of going in. Just don't wait, go."
Cave and her partner, the comedian Alfie Brown, are also parents to Donnie (5) and Margot (3).
Brown now has the virus.
"It's such a strange illness and it affects everyone completely differently, and so it's completely affected our lives over this period of time," Cave said.
"He's OK, he's doing much better now.
"It's just bizarre how it didn't affect me and it's completely affected him.
"We know people who have had to go to hospital.
"Everyone has said to me, 'I didn't think babies can get it', but it can affect everyone.
"Because of the stress, my milk dried up a little bit and now I've had to give him formula."