Infected are quarantined, and their posessions burned. They've been given food, water, and weapons, and can live no less than ten miles from Avalonian borders, citizens, or military installments, until a doctor can give them a clean bill of heath and prove hat they are not carriers of the disease.
Avalonia takes the plague to be a bad omen or something, and so any men to have contact with the infected trail ten miles behind the main rank of soldiers. The infected trail another ten miles behind those men. Needless to say, the invasion is canceled, and the men are returning to the Avalonia area.
Avalonia will have the whole invasion force quarantined outside of the borders in separate camps as defined above, communicating via smoke signals, until the sick die (despite the greatest efforts of Avalonia's doctors), or all heal as defined by a skilled physician.
Avalonia resolves to maintain more sanitary conditions nationwide. A medical college is being constructed so that in the event of a plague, or general widespread sickness, something can be done.
More troops are being trained so that in the event of an army-wide plague, Avalonia will have some means of sefense, however meager.
Any trade with Avalonia is suspended for one week due to the sickness, and citizens are advised to avoid contact with their neighbors for the time being.
Avalonia is on lockdown.