The Plague (no official name given) is a virus that affects only characters with powers. The Plague first destroys their power and eventually kills them, much like Green Kryptonite does to Superman.
Plague History
Before the existence of the Plague was officially announced by Mohinder Suresh in the episode Four Months Later... (2-01), the Plague had already been mentioned several times.
The first time the Plague was mentioned in the Heroes TV show was in connection with Shanti Suresh, who is the first known hero1 to have died of it.
Later, Molly Walker was cured of the Plague by Mohinder by using his own antibodies.
Maya Herrera is host to the Plague and not only acts as a reservoir, but also a method of transmission of the Plague to people without powers.
Cure
The only known cure for the Plague are Mohinder's antibodies. However, there is only one Mohinder Suresh, and he would be unable to produce enough antibodies to cure all those infected in a pandemic.
Alejandro Herrera has the capability to "absorb" the virus and save those affected if he acts within a short period of time.
Method of Transmission
Excepting Maya Herrera, the method of transmission for the Plague is unknown.