Becoming infected...what happens to magic? Options

[Machiavelli]

The rules say:

Magic and Essence
Starting Infected characters start with Essence 5 and Magic 1
during character generation.

But what if you become infected during gameplay?

Besides that, i really don´t understand the following rule, especially the bold section. What situation could cause this effect?

For Infected characters with Essence Loss, Magic loss does
not occur every time they lose or spend a point of Essence, and
their maximum Magic attribute is equal to their current Essence
+ Initiate grade. If their maximum Magic attribute is lower than
their current Magic attribute, their current Magic rating is adjusted
down to the reduced maximum. These points are lost and
must be repurchased with Karma. Infected characters can temporarily
exceed their current Magic maximum by spending Essence
points (see Essence Drain, p. 288, SR4).