"mild disease" is correct and usable in written English.
For example, "The patient was suffering from a mild disease."
ABO incompatibility usually leads to mild disease.
(Women with mild disease had less).
"When adults get vaccinated," Dr. Van Rie said, "they protect themselves from a mild disease.
Chickenpox is a mild disease for the vast majority of kids who catch it.
Most of the time babesiosis is a mild disease.
Could this typically mild disease make this woman this sick?
Q FEVER -- The relatively mild disease can produce crippling chills, coughing, headaches, hallucinations and fevers up to 104 degrees.
There are many similarities here with Rubella – also a very mild disease but able to induce birth defects.
Accordingly, infections range from mild disease to a potentially deadly illness.
The logic behind giving shots to those who are not as vulnerable (children generally experience flu as a mild disease) is part of a concept called herd immunity.
Initially it caused only mild disease that escaped detection, but by 1997 the virus had mutated to a deadly form that could kill chickens within 48 hours.
