Every day we interact with objects, places and sensations that affect the way we think and feel. This can be used to the writer's advantage by planting symbols in the reader's path to reinforce a specific message, feeling or idea.
Look at the setting and the character's state of mind, and then think about what you want the reader to see. Is there a descriptive symbol or two that works naturally within the scene to help foreshadow an event or theme, or create insight into the character's emotional plight?
Oily, brackish water
Stunted, gnarled trees
Fruit covered in scabs or pock marks
Patches of fur missing
An inability to fly straight (birds)
Black rot (crops, grass, plants)
Curled up leaves, browning at the edges
Tree rot, dead limbs
Unexplained dead fish/aquatic creatures on a shoreline
Spindly animal legs, weak, erratic walk
Animals foaming at the mouth, panting uncontrollably, staggering, stumbling
A yeasty, decaying smell
Medic alert bracelet
Get well flowers/cards
Sweating & feverish
Sallow or yellowing skin
Shortness of breath
Swollen lymph nodes
Mental illness (Paranoia, memory loss, depression, etc)
Loss of appetite
These are just a few examples of things one might associate with Illness. Some are more powerful than others. A hospital is a strong symbol, and likely will not require reinforcement. However, a pill bottle may not foreshadow illness on its own. Let the story's tone decide if one strong symbol or several smaller ones work the best.