The thorn on a rose stem provides an excellent device for injecting infectious material into your skin. Any of the treatments you use in your garden for the health of your roses can provoke an inflammatory reaction when injected by a scratch or prick. So can various bacteria and simple dirt. One fungus, Sporothrix schenckii, is notorious enough for this mode of infection that it's recognized as the cause of "rose-picker's disease."
The Thorn Itself
If a thorn breaks off in your skin, and especially if it has gone straight in and deep, keep an eye on the wound to make sure it stays clean and heals promptly. If pain, redness or swelling develops, or if the thorn remains embedded in your skin, seek medical assistance. Even a small infection can form an abscess that will need to be lanced to release excessive material your body sends to fight the infection.
The Sporothrix fungus that lives on rose thorns cannot live on human skin or nails, where other fungi may have little or no effect on the host's comfort or health. If Sporothrix is injected into the skin by a prick, however, it can transform into a yeast and spread beyond the original wound.
The Most Common Infection
Most often, when Sporothrix gets into skin, it causes a localized but potentially chronic problem. At the wound site, it can cause ulcers that are not only red, but swollen with pus until they rupture and drain. It can also spread back through the lymphatic system that sends the pus against infection, causing pain and swelling in lymph nodes up the arm from the original wound.
If a thorn happens to inject Sporothrix into the knuckle of your finger or your elbow, it can cause an arthritic infection that is very painful. It can also infect the eye and surrounding delicate tissues. Most rarely from rose-thorn pricks, sporotrichosis can become a systemic, or body-wide, infection, including the central nervous system. The fungus also grows in sphagnum moss, from which it can be inhaled.