It's in terms of compatibility... some are less severe because
the human's immune system can go against it, some new strains are
stronger because the body has not met it before, and when the first
time they meet each other, a blind battle will start, and either
side have a 50-50 chance of winining...
But with mutation and all, the bug may become too strong, being
resistant to antibiotics and the human's immune system and once it
defeats them, they take control over the body and start to divide
and become more (just like AIDS)... The continuous mutation is
called 'antigen drift'...
If a battle is fought against a common bug (which once infected the
person making him ill, but he recover and develop antibodies to
protect him from future battles), the battle is a sure win for the
human body...
