According to the American charity Dr Kent Brantly was working for, Samaritans Purse, he took a "turn for the worse" whilst being held in isolation.
He is a medical director for the organisation and became ill whilst working in a care centre in the Liberian capital of Monrovia.
He's currently being given 'intensive treatment' in isolation at a specialist hospital.
His wife Amber said in a statement: "We appreciate so much all the words of comfort and acts of kindness extended to our family.
"As people with a deep faith in Jesus, we sincerely thank the thousands of people worldwide who have lifted up Kent and this dreadful situation in prayer. We continue to lean on that faith and take great consolation in our God in these times.
"We have a strong family unit within a stronger faith community that has given us incredible support. Kent remains very physically weak but his spirit has been determined throughout this ordeal.
"This is a challenging time for our family."
Dr Brantly's colleague Nancy Writebol is also ill with the virus. Her condition is also 'stable but grave'.
Samaritan's Purse President Franklin Graham said: "We are doing everything possible to help Dr. Brantly and Nancy.
"We ask everyone to please pray urgently for them and their families.
"Their heroic and sacrificial service—along with the entire team there—is a shining example of Christ's love in this crisis situation."