This flail boasts a bright red spiked ball at the end of its chain.  With a successful DC 15 Intelligence (Medicine) check, a creature can discern the spiked ball to be a model of a virus.

You have a +1 bonus to attack and damage rolls made with this magic weapon.

Virulence. This weapon is plagued, and attuning to it afflicts you with the disease it carries. When you hit a creature with an attack using this weapon, the target must succeed on DC 15 Constitution saving throw against disease or contact crown cough.

When you attune to this weapon, the version of the disease you contract is asymptomatic and can't be cured or ended until your attunement to the flail ends.

Crown Cough

Crown Cough is a generic term for a broad category of highly infectious respiratory viruses, known to healers as coronal viruses. Though crown cough spreads easily and quickly, its symptoms tend to be relatively mild in all but the most severe of cases.

Symptoms manifest 2d8 days after infection, and vary drastically from case to case. An infected creature suffers 1d6 symptoms. The first symptom is always a cough, with any additional symptoms determined randomly by rolling on the Crown Cough Symptoms table, rerolling any duplicate results.

Any creature that starts its turn within 5 feet of an infected creature whose symptoms have manifested must succeed on a DC 15 Constitution saving throw or also become infected with the disease. Creatures that don't need to breathe are immune to the disease.

At the end of each long rest after symptoms have manifested, an infected creature can make a DC 14 Constitution saving throw. On a successful save, the DC for this save decreases by 1. When the saving throw DC drops to 0, the creature recovers from the disease. A creature that fails three of these saving throws gains an additional symptom from the Crown Cough Symptoms table, ignoring any result of 1.

Crown Cough Symptoms

d12	Symptom
—	Cough. The infected creature must make a DC 11 Constitution saving throw at the start of each of its turns. On a failed save, the creature begins coughing uncontrollably and is incapacitated until the start of its next turn.
1	Asymptomatic. The infected creature becomes immune to the effects of all other symptoms, including cough, but can still pass the disease on to other creatures.
2	Anosmic Ageusia. The infected creature automatically fails Wisdom (Perception) checks that rely on smell or taste.
3	Body Aches. The infected creature's movement speed is reduced by 5 feet, and it has disadvantage on Strength- and Dexterity-based ability checks and saving throws.
4	Chills. The infected creature gains resistance to fire damage, but is constantly subject to the effects of extreme cold (as described in chapter 5 of the Dungeon Master's Guide) regardless of current environmental conditions. If the creature also has the fever symptom, the effects of the two symptoms alternate randomly whenever the creature completes a short or long rest. Roll any die. On an odd result, the creature suffers chills. On an even result, it suffers fever. While one symptom is active, the other is suppressed.
5	Delirium. Whenever the infected creature rolls a 1 on the d20 for an attack roll, ability check, or saving throw, it immediately suffers the effect of the confusion spell until it succeed on its saving throw against the effect (save DC 15).
6	Diarrhea. Whenever the infected creature completes a short or long rest, it must must make a DC 11 Constitution saving throw. On a failure, the creature requires the use of a restroom every 2d10 minutes for the next 1d4 hours, during which time it can't complete short or long rests. A restroom stop requires 1d4 minutes, and can overlap with other restroom stops. During a restroom stop, a creature is incapacitated, has a speed of 0, and becomes the center of a stinking cloud for the duration of the restroom stop. The DC for the spell equals 8 + the infected creature's Constitution modifier + its proficiency bonus, and the infected creature automatically saves against its own stinking cloud. In addition, the creature must drink twice as much water each day (a minimum of 2 gallons) to avoid exhaustion (see "The Environment" in chapter 8 of the Player's Handbook.)
7	Fatigue. Whenever the infected creature finishes a long rest, it must succeed on a DC 11 Constitution saving throw or gain one level of exhaustion, to a maximum of 4 levels. Levels of exhaustion gained this way can’t be removed until the disease is cured.
8	Fever. The infected creature gains resistance to cold damage, but is constantly subject to the effects of extreme heat (as described in chapter 5 of the Dungeon Master's Guide) regardless of current environmental conditions. If the creature also has the chills symptom, the effects of the two symptoms alternate randomly whenever the creature completes a short or long rest. Roll any die. On an odd result, the creature suffers chills. On an even result, it suffers fever. While one symptom is active, the other is suppressed.
9	Headache. The infected creature has disadvantage on saving throws it makes to maintain its concentration.
10	Nausea. Whenever the creature eats or drinks, it must succeed on a DC 13 Constitution saving throw or regurgitate everything it has consumed since its last short or rest.
11	Sore Throat. The infected creature suffers disadvantage on checks that require speaking, and must make a DC 13 Constitution saving throw when casting any spell with a verbal component. On a failed save, the spell fails and is wasted.
12	Respiratory Failure. The infected creature's lungs begin to deteriorate. Whenever the creature completes a short or long rest, it must succeed on a DC 15 Constitution saving throw or begin suffocating, as described in chapter 8 of the Player's Handbook. A suffocating creature may repeat the saving throw at the end of each minute, ending the effect on itself on a success.

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Proficiency with a Flail allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Damage, Combat, Sap