This post comes from the first draft I ever posted to the site back in April of 2006. Originally, this was designed for the Action! System, but I have changed it to be fairly system agnostic.
The phage is a complex magical disease that gives the infected person some amount of magical resistance.
Every week the person is alive, 25% of their hit points (minimum of 1 ) are claimed by the disease. When the Phage points are greater than the hit points, the infected person dies.
At any point, an infected person can ‘give’ their phage points to a healthy person, thereby reducing their phage points to 0. The creature receiving the phage points has the same number of hit points taken from them.
If the receiving person takes less than 50% of their total hit points in phage, they will heal the points (one per day) with no ill effect. After six times of being infected with less than 50% of their life points, the target will become immune to the phage. After a target is immune to the phage, he/she will no longer be able to take any phage point damage.
If the receiving person takes more than 50%, but less than 100% of their total hit points in phage damage, the victim contracts the phage, though it won’t appear until after the initial points heal.
For example – Tom has the phage and 40 hit points. Rick has 56 hit points.
Tom has 20 phage points because it has been building for two weeks. (25% of Tom’s points per week or 10 points per week.) He ‘gives’ his points to Rick through skin contact. Rick takes 20 hit points of damage. He will heal one point a day. Because the 20 points of damage was less than 50% of his total 56 hit points, Rick will not be infected with the phage.
Example 2 – Tom has the phage and 40 hit points. Rick has 56 hit points.
Tom has been building for three weeks and has 30 phage points. He gives them to Rick. Rick takes 30 hit points of damage. Rick has the phage now because the 30 points is more than 50% of his total hit points (28). However, he will not show signs of the disease for some time. First, the 30 points will heal (1/day). Then he will have the phage, but no phage points will accumulate for the first week. During this time, Rick will know that he now has the phage. After one week, Rick will then collect 14 phage points per week. (25% of 56 is 14).
There is no cure for the disease despite being able to make others immune to it. The victim of the disease can only prolong his/her life. Letting the disease take its course will kill the victim in one month.
The physical symptoms can be whatever you like. I tend to think that benign magical effects work well like shifting tattoos, bumps, blurred vision, etc.