Here's what I don't understand... If viruses and bacteria are all over your body and do some useful things, how can some be bad? What separates 'bad' virus from a good virus?
{ edit: explanation } Viruses are tiny fragments of dna and proteins that are only there to throw a monkey wrench into your cells. Sometimes that monkey wrench does fairly harmless things. These viruses thrive, because they find lots of healthy hosts over a long period of time.
But some break things badly, reproduce out of control or change your cells' functions to be less optimal. These don't do as well, since their hosts don't thrive.
The viruses don't 'care' what they're doing; they're just bits of junk that mutate a lot. The ones that kill things quickly, tend to die out.
The only reason the world isn't full of horrible murdering viruses (any more than it is) is, statistics. Not some grand plan or kindly act of nature.
Micro organisms are all about spreading genetic material, since they are so small, they have evolved passive strategies to do just that. Such as using a host organism and hitching a ride via fluid exchange.
What makes all the difference is where they will end up, and how they behave in particular environments. Coronavirus outside your body is harmless, but if it gets in your lungs, it may destroy those while it replicates.
Your gut teems with bacteria. There are dozens of species living in your colon. And you actually need them since they help break down food into its constituent parts in order for your gut to be able to absorb the nutrients.
https://en.wikipedia.org/wiki/Human_gastrointestinal_microbi...
However, if your bowels get perforated and that bacterial contents seeps between your organs, you'll get in deep deep trouble: these bacteria will start to infect organs they shouldn't infect, get into your bloodstream. You'll get sepsis and risk death.
So, harmless bacteria turn deadly if they end up in the wrong spot.
The interesting part about viruses is that through the way they operate - injecting itself into the DNA of our cells which will then bang off new copies of the virus - their genetic material has embedded itself into our own genome. It is estimated that 5 to 8% of the human genome consists of "endogenous viral elements". These are basically viral left overs, not viruses themselves, to be clear!
https://en.wikipedia.org/wiki/Endogenous_retrovirus
Micro organisms have been with us for a long, long time. We have evolved concurrently with them. And we will keep evolving alongside, evolving strategies - such immunity, or our ability to understand how they work - to ward off infections while new species will pop up and some might be successful in thwarting our evolved defenses.
That's not the purpose of the berry. The purpose is just to ensure the survival of the seed. In drought-stricken environments, the seed is probably better off keeping its shell of water and fructose until it can form a root system.
For other plants it may be advantageous to be poisonous to one species and not another
Birds tend to travel farther during any given time period than (typically terrestrial) mammals do, making them better at spreading seeds over a wide area. So it's evolutionarily advantageous for plants to optimize their fruits to be eaten by birds instead of mammals.