– Sometimes plants do get sick, if you see a dead leaf or area, cut it so it won’t get to the rest of the plant.

– They don’t have enough sunlight (indirect or direct). Move them to a brighter / darker place.

– It’s too hot for them, they mostly do not like being near heating air conditioners.

– It’s too cold for them, consider bringing them to your room.

– If they have dust on their leaves, it helps to clean them so they can get more sun.

– For water plants, if you see something is wrong, change its water.

– For soil plants, or they have too much water, or too little.