Obtain the cure.

It is not easier to do something if you know it can be done. But it is a lot easier to keep trying, try harder and pour resources into the endeavor if you know it can be done.

There are going to be cases of this dread disease on both sides. The people who invented it are going to catch it. Even if they are very careful, both sides have the disease now. If I have people dying of the disease I have plenty of fomites / spores / whatever to load into shells and send back.

The inventors are going to have to ramp up their cure efforts to keep their soldiers fighting and make it available along their front. They will want the cure as close as possible to the front lines to keep well people well. That offers the possibility to capture the cure from its inventors. Or capture soldiers and ask them if they were treated with the cure; a soldier will not know molecules but can describe a pill, a shot, a soothing unguent etc. Or best: make it known that your side will pay handsomely for samples of the cure and count on a deserter to bring you some.

There was enough science in the 1910 to work on a project like that. A treatment that could cure infection would be one of three things: chemotherapy like Salvarsan as was being used against syphilis, or immune based therapies which would be either a vaccine or plasma from immune animals or individuals.

Obtain the cure.

It is not easier to do something if you know it can be done. But it is a lot easier to keep trying, try harder and pour resources into the endeavor if you know it can be done.

There are going to be cases of this dread disease on both sides. Both sides have the germ now. If I am attacked with germs, then I have people dying of the disease I have plenty of fomites / spores / whatever to load into shells and send back.

The inventors are going to have to ramp up their cure efforts to keep their soldiers fighting and make it available along their front. They will want the cure as close as possible to the front lines to keep well people well. That offers the possibility to capture the cure from its inventors. Or capture soldiers and ask them if they were treated with the cure; a soldier will not know molecules but can describe a pill, a shot, a soothing unguent etc. Or best: make it known that your side will pay handsomely for samples of the cure and count on a deserter to bring you some.

There was enough science in the 1910 to work on a project like that. A treatment that could cure infection would be one of three things: chemotherapy like Salvarsan as was being used against syphilis, or immune based therapies which would be either a vaccine or plasma from immune animals or individuals.

Medical science is not that exciting for a war fiction. More exciting is more war. Once the side without the disease captures and can make samples, that side can take germ warfare up a notch using techniques developed in later wars. Attacking crowded cities and civilian targets is something the inventors of this disease are not doing, according the the OP. But it would be a more effective use of germ warfare than painting it on bullets, and a surprise. By the time the disease makers understood what had happened they would have a scramble on their hands trying to contain the damage.

The biplane crop duster is pretty cool, but ideally this would be done covertly, and the epidemic in their cities blamed on the authorities of that country.