Fungus can grow inside a lens if it has moisture being left there for a long time. However, fungus can easily be killed if they got expose to sunlight. That said, you should use your lens regularly (every week) and rest assure you shouldn't have fungus. Shoot an object a day, keep the fungus away ! hahaha
Unluckily, one of the lens my brother own has fungus in it and I have decided to give it a try to remove it.
I shall warn you DO NOT FOLLOW WHAT I WILL SHOW YOU BELOW. DO NOT ! DO IT AT YOUR OWN RISK !
This is how fungus looks like on my brother's Konica Minolta 75-300mm f4.5-5.6
Step 1: Remove the filter
Normally lens will have a filter to protect the front element of the lens. Remove that by turning it anti-clockwise
Step 2: Look for the notch
Look for a notch as shown below around the side of the lens after you remove the filter.
Step 3: The tools
With this tool below, I will turn the ring holding the front element anti-clockwise to remove it. This is just an alternative tools, there are more 'suitable' tools design to do this. I have use a cotton to protect the front element to prevent scratching it
Step 4: Remove the front element
Now that you have remove the ring, you can continue to removing the front element. I turn the lens upside down to remove it.
Step 5: Clean it
Now that you have access to the fungus part, you can continue to clean it. I have used some cotton buds and VCR tape cleaner.
Finally. Clean as new !