Corona

Also see Vaccina

Token Upgrade
You can use a pet token to multiply all her bonuses by -1. (This will turn her -30% into 30%)

= The Corona Quest =

Disclaimer
By unlocking Corona you are agreeing to start her questline. This quest cannot be canceled and, even after Corona and Vaccina are evolved, there is still an upkeep to consider. In actuality, this quest is not particularally difficult but starting it without fully understanding the ramifications could lead to prolonged harm for your pets. If you are confident on this matter, than continue reading.

Preparation
Go to the Settings tab and enter "Corona" into the text field. This will make Corona and Vaccina visible on the pet page. Ensure you have the following before proceeding:
 * A GC capable of growing a 20K evo requirement
 * 1000 Bound Feathers
 * 750 Special Wood
 * 200 Cure Potions (300 Recommended) Cure Potions. Your Alchemists can craft these for 2 Health Potions and 2 Herbs each.
 * A supply of Health Potion Xs

It is also worth noting that the more pets you actually have, the faster it is to get the 100 sick pets that is required.

Unlocking Corona/Vaccina
The next step is to use a pet token to unlock Vaccina. Doing this will grant the ability to unlock Corona. You may choose to unlock Corona immediately or wait for Vaccina to reach her growth evo requirement. As Corona requires you to have had 100 pets that have been sick before (a cured pet getting sick again does count for the total), it is recommended to unlock Corona immediately after getting Vaccina. This is because the sick pet requirement can take a long time to achieve and it's better to start it as soon as possible. While you are waiting on the sick pet requirement, be sure to get the 20K Growth requirement on Corona, and the 15K Growth requirement on Vaccina.

While Corona is unlocked, she will spread a sickness with a 5% chance to make a new pet sick every hour. Every sick pet also has this same 5% chance (this is good). Every sick pet will loose 1 point in growth every hour, down to the minimum of 500 growth total (Keep in mind that if you are feeding your pets a few times a day, than the growth loss from the sickness is completely negligible and not something to worry about). Using a cure potion on a sick pet will remove the sick status and it makes the pet eligible to get sick once again. This is done with the Cure button within the pet info screen. You can tell if a pet is sick if there is a green sickness icon over the left side of the health bar. This icon can also be seen at the top left of the pet's info screen as well.

The Sickness icon:

Getting 100 Sick Pets
The next step is to wait. Now remembering that each pet has a 5% chance to cause another pet to get sick, the fastest way to get the 100 required pets is to wait until the grand majority of your pets have the sickness status, and then only cure a small handful at a time. I waited until all of my pets were sick (Corona cannot get sick herself) and then cured a few. I then waited another hour and did it again. You can do this until you are ready to cure all of your pets for the 100 sick pets requirement.

It is also good to know that the pets that are eligible for the sickness are the ones at the top left of the pet page. If a pet is already sick, the candidates are the next pet(s) in line, going from top left and moving right across the rows.

After you have achieved the 100 sick pets, you can evolve Corona into an Adventurer. This, in turn, will allow you to evolve Vaccina into an Alchemist (Be sure to have at least 100 Cure Potions for her evo). Vaccina's evolve has the unique ability to create Vaccines (2 Cures, 5 Health Potion Xs, and 5 Herbs). Give Vaccina one of your best Alchemy Pots and put her to work making Vaccines. Giving the Vaccine to a pet will make them entirely immune to getting sick. Keep in mind that Vaccines do not cure a pet that is already sick.

Moving Forward
From here on out Corona's sickness is still a factor to consider. It is recommended to have a supply of cure potions, and vaccines in stock to prevent future pets from getting sick.