"Good Morning America" co-anchor Robin Roberts will today undergo a bone marrow transplant to treat myelodysplastic syndrome, or MDS, a disorder in which the bone marrow fail to produce enough healthy blood cells.

Dr. Gail Roboz, who is helping Robin prepare her for her bone marrow transplant, appeared again on "GMA" today to discuss Robin's treatment.

Click HERE to Follow Robin's Journey.

"Everything is going great," Roboz said. "Medically speaking, everything is going fantastically."

In preparation for the transplant - her sister Sally-Ann Roberts, who is a perfect match, was the donor - Robin underwent extensive chemotherapy. First, she received the "good" chemo, which boosted her blood cells and marrow to get her ready for the next phase, Roboz explained.

"We could see in watching Robin over the summer that she looked fantastic," Roboz said. "She was having an easy time with it. That was really MDS-directed treatment. That was to mow the lawn, get rid of as many MDS cells as possible, boost the bone marrow and get ready for the transplant."

After that, the chemo Robin received was very different. For three days last week, she had chemo every six hours for two and a half hours. On Tuesday, she underwent 18 hours of uninterrupted chemo, which decimated her marrow and immune system. For now, she has no resistance to infection.

"This type of therapy over the last week has been much more intensive," Roboz said. "This isn't just mowing the lawn and not getting rids of the weeds on top, it's digging down deeper and really trying to empty out the bone marrow cells and get rid of the immune system cells so that the new ones from Sally-Ann can come on in."

"Robin looks great," she said. "She's a powerhouse but she feels crummy. Her mouth hurts. She's got a headache that won't quit. Nothing tastes right. It's hard to get up and even move around in the room. This is someone who's used to 50 hours a day and an athlete with tremendous stamina. It's powerful to hear her say that reading a few emails or sitting up in bed is a lot of work."

Click HERE to read the latest message from Robin

The transplant itself will take between 30 and 60 minutes, and when it's done, Robin will be kept in room designed to keep the air as clean as possible - but she will not be totally isolated. Medical staff, friends and family will be able to visit, Roboz noted.

To find out more about bone marrow donation and sign up for a registration kit from the Be the Match Registry, click HERE.

Even though Robin's sister was a perfect match, Robin's system will still try to attack the donor cells. Doctors will work to prevent any symptoms of graft vs. host disease - a rejection of the bone marrow transplant.

"This is the beginning of the rebuilding phase," Roboz said. "Rebuilding is not immediate. It takes weeks to months…These cells have to swim around. There are millions of them being infused today into Robin. They have to find out where they want to go and they have to rebuild her entire blood-forming and immune system."

Robin's time in the hospital will be determined by how well her body is adapting to the transplant. The markers for recovery typically at 30 days and then 100 days post-transplant.

"Thirty days gets us to we hope that we see nice, normal blood counts," Roboz said. "We hope that we can stop supporting her with things through the IV because her systems are up and running but it still doesn't mean she feels like Robin again."

"It's still very early," she said. "We're hoping that at that point approximately she'll be able to get out of the hospital but there's still a long process after that of fully regaining her strength and fully regaining all of the power that she needs to be back here."

While Robin has the support of #teamRobin behind her, she is undergoing the procedure without her mom by her side, something that Roboz said has been difficult. Lucimarian Roberts, Robin's beloved mother, died Aug. 30 at the age of 88, just before Robin began her treatment.

"I think she misses her mom a lot and I think it's been really hard to go through this," Roboz said. "She said yesterday this is the first hard thing that she's had to go through without her mom. So I think that's been a real challenge but she is a trooper and fighting through it and doing very, very well."

Roboz has answered questions the "GMA" staff and fans had about Robin's treatment, procedure and recovery process. Read her edited answers below.

How will Robin feel?

She'll be tired, nauseous. But this is not the transplant procedure itself. This is not painful or hard to go through. After that's done, your body has a wiped out immune system. Sally Ann's cells will be trying to set up shop to set up an immune system and marrow to fight off infection. It takes time. You're completely vulnerable to infection and need to be on antibacterial, antifungals, and antibiotics. Your body is colonized with germs and even exposure from years ago can be dormant in your system.

The day a patient receives a bone marrow transplant is often called "Day Zero." What are the benchmarks in Robin's progress from there?

Days 0 - 30: The first 30 days are when the new cells are setting up shop and getting ready to regenerate. So many of my patients ask me, when the catheter is placed in her chest for the transplant, 'How do these cells know where to go? How do they find the bone marrow?' These cells have homing signals to help them find where they need to go and start growing and creating an environment for themselves.

…

"Early recovery" is when the bone marrow is taking hold, and blood is being created normally.

[Until her donated cells engraft and] she starts making blood cells and platelets on her own, she's completely dependent on platelet and blood transfusions. It's about filling in for what your system can't do. It can't fight infections. The medications have to fight everything that's out there. You can't make cells or platelets, so you'll have transfusions. You can't eat, so we give you something to keep you nourished.

….

This is a tough time in the treatment. There are mouth sores, weight loss, diarrhea, food doesn't taste good, there's hair loss.

What will her hospital set-up be like during this period?

The team is there the whole time, monitoring how you're feeling, what your experiencing, constantly. It's not the "intensive care unit" but its intensive care.

What will you be looking for and monitoring in the first 30 days?

We need to get through that period of wiped-out blood counts. We'll start to see white blood cells, red blood cells and platelets growing on their own. That's the evidence that the graph is working. That's when we breathe our initial sigh of relief. She'll need fewer transfusions, fewer medications, she'll be getting out of the hospital [after 30 days]. At that point, she'll still be on significant meds to regulate her immune system, but she'll be recovering at that point.

Talk about Days 30 to 100.

Starting at that 30 Day mark, she'll have bone marrow evaluations to see that it's growing normally.

In the days that follow - until the 100 day benchmark - we're seeing problems getting solved. She'll get off the meds, she won't feel as tired or as gross as you do when you're on so many pills. She'll start liking to eat again. She'll have to remind herself what she likes to do. You're separated from all that stuff for so long, you have to slowly reintroduce yourself to what you used to do.

Will she want visitors?

Probably not every day. It's a day to day thing. Well-wishers have to understand that the patient drives what's best for them. Sometimes it's best to let someone sleep.

When does she go home? If all goes well, Robin will go home after 30 days.

When she goes home she won't be feeling like herself, but we hope she'll be able to do some exercises, read and focus. It's important not to put a time stamp on anything. People are variable. Especially the way people are micromanaged in this process. You can't be worried if you're slower than others.

When will she feel like herself again?

Transplant is an individual experience. …We can't tell you how and when someone will feel better. Some people say they didn't feel 100% for a year. Others are ready to go back out much earlier.

What happens after the 100 Day Mark?

We have to watch patients super-closely for 100 days. We don't trust anything for the first 100 days. After that, we're happy. With cancer patients, we use five years as a benchmark to a cure. When someone has had a marrow transplant, we'll be watching her for life, to see if she's having symptoms.

How will you monitor when Robin will be able to come back to work?

We'll need to see high blood counts that don't require transfusions. She must be totally off antibiotics. Feeling sharp. There's an extra high bar for her because she has a harder job than most. She needs to have her system all up and running - marrow, immune system, nutrition - everything that was wiped out in chemotherapy has to be restored.

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