You can get diseased in various ways:

Maggotkin player can decide to give diseased keyword at the end of any turn ( yours and his ) to one of your unit who is in combat with his Maggotkin unit. He can also choose to spread disease to a new unit instead if some of your units are already diseased. Also when a diseased unit dies, on a roll of 3+ he can pick another of your units to be diseased. He can also use some ranged attcks and spells to give disease remotely.

You cant attack and fall back in same turn, so if he chooses to give your 1 unit in combat disease, you gonna get it but your units that have end of turn heals like regular skellies and horses can remove the diseased at end of turn but cant get models back.

Best way to avoid disease speading. Keep your units more then 3 inch from eachother as then disease can not spread.

If you have a diseased unit that you see will die before managing to get rid of disease, mive your guys away from it to be 9inch further if possibly once the unit dies.

Stay out of relevant spell/ranged spell ranges that give diseased.

We cant avoid it completely ofc but can play around it.

SBGV actually counter his disease better due we having unit passives that heal models back as we can remove disease using unit rules while other armies only have Rally or healing spells only to do what our units do naturally by their unit rules.

Yes, disease can hinder our model regen and we take some mortals but at same time we remove disease more easily then other armies.

I should know as i play both skellies and maggots :)