General synonyms:
* Sickness
* Illness
* Disease
* Ailment
* Malaise
* Unhealthiness
* Pathology
* Mortality
More specific synonyms:
* Death rate (if referring to the rate of death in a population)
* Incidence (if referring to the number of new cases of a disease)
* Prevalence (if referring to the proportion of people in a population who have a disease)
* Suffering (if emphasizing the emotional and physical distress caused by illness)
* Degeneration (if referring to a decline in physical or mental health)
* Pathogenicity (if referring to the ability of an organism to cause disease)
Formal synonyms:
* Morbific
* Morbid (though this word can also have a negative connotation, suggesting an unhealthy interest in death or disease)
The best synonym to use will depend on the specific context of your writing or speech.