[Leica] What To Do With A Lens With Fungus?

[robertbaron1 at gmail.com (Robert Baron)]

My question involves an old Nikon lens but I tend to trust the answers I
get from the LUG more than from other groups.  I have an old Nikon 20mm
lens I sent to KEH for sale.  They sent it back with a note saying they had
found some haze in it.  I then sent it to MPB and they responded:

Unfortunately, due to your Nikon AF 20mm f/2.8D having fungus present
within the optics we are sadly unable to purchase this item from you at
this time. This can be seen whilst inspecting the lens? optics with a
torch. Moderate amounts of fungus can be detrimental to image quality, but
can also spread to other equipment given enough time. We will return this
item to your collection address on file.

It can spread?  What should I do with it, toss it in the trash?

--Bob