How Fungi Can Be Harmful Or Helpful?

Harmful-it damages food products,paper,leather,paint and textiles.
  -causes disease in animals and plants which humans are interested in.
  -causes disease in humans,either directly or by toxins
  -contains poison which is deadly to humans,some mushrooms.
Helpful-major source of pharmacologically active drugs (antibiotics)
  -act as antifungals
  -act as statins
  -used to culture foods like yeast and for fermentation
  -medical (clinical) use,cancer treatment
  -edible; some mushrooms
  -used to control pests by competitive exclusive principle
  -helps in bioremediation ;turning heavy fuels and other componds into CO2 ,water
  and  basic elements.
  -used as model organism;easy to culture and manipulate
  -industrial chemicals eg malic acid
  -industrial enzymes eg lipases

They can give us a source of food like mushrooms.
A yeast fungus called Saccharomyces cereviseae is used to make the alcohol in beer.
This same fungus is used when we make bread--without its help, we would have flat bread. Without fungi, we would have piles of trash everywhere because fungi get food from our trash. They eat the trash and make it into soil. That is why we do not live in a landfill!
Harmful are fungis that produce some type of poisonous chemicals. Like there is many mushrooms that could kill you if eaten.
Source(s):
There is many other information about this. If you research well you can understand more about fungi.
O by the way I'm not going to drink bear after is made of some nasty fungus.

It can be harmful as it grows on food stuff like bread or on leather rotting it.it can even cause diseases espeacially skin infections.its useful as it is edible eg mushrooms,produces antibodies like penicillin.