Say you're a single celled being with the following characteristics.
Able to control disease based cells around it
You've been around for millennia. You've "possessed" humans many times over, using their bodies to interact with the populace and whatnot. In essence, you kill them and use their body for yourself, essentially replacing either from birth or from some point in their life. You've generally laid low even in your human actions. You've always known of humanity. In fact, in your birth you obtained the memories of a human linked to you. Naturally, you're rich, heavily invested in politics, and tend to operate in the shadows. Lots of people know of you, but don't tend to realize that you run everything in your country aside from being an elected official.
You decide getting elected, being rich, etc. just won't cut it anymore to satiate your desire for control. So you decide you want to make your grand reveal. You want to make yourself the body of a literal giant. Something massive. You'd use the bodies of humans but you don't think that's flashy enough. So you decide you want to use disease as the cellular structure of this monstrosity. After all, it's what you were born with control over! So you decide to make a monster with the following attributes:
Roughly a mile tall with either the appropriate strength for that size or greater strength. Bigger and stronger is always better (as you can discard anything you don't need), but you definitely don't want to be treating the ocean like a kiddie pool.
(Possibly confusing bit) Since bacteria and diseases are single cellular you also want your creation to be able to disperse itself into the air and be able to control local air currents. Essentially you want it to be able to take the form of an artificial funnel cloud without being utterly destroyed. This is definitely something you want.
Durability - you don't want it falling apart 24 hours later and the angry locals killing you.
Now you aren't afraid of nuclear missiles or anything like that. You have the access codes. You can easily dispose of them. Of course, that doesn't mean you don't want to have safeguards in case some were lying around. You've seen nukes be used in wars before. You know your new body would have to much much bigger to survive the immediate blast and make your escape.
You plan to primarily grow your disease cells by infecting the population with them. You want some kind of plague or pandemic to produce enough cells to let you grow. Preferably they would be natural cells or appearing natural. See, people once knew of you. There are (incredibly minor) deities and other beings that if even suspected you might be alive, would strike you dead on the spot. They think you're dead and cannot even remotely suspect you're trying this. Of course, if you succeed, they won't know what hit them. However, you have to do this in secret. Otherwise, your plan will fail.
Based on your other political actions and everything starting to converge towards you being removed from power, you only have roughly a few months to a year to execute this plan. The faster this can be done the better. Chances are, any transfer of body will just result in a loss of power due to things becoming more distributed. Nothing dictator like. Just the normal things such as companies choosing to split apart, reelections, etc. It has to be you that pulls it off (otherwise you risk losing control of the nukes and other things that might be able to kill you at full strength).
So the question really is:
What diseases do you use to create this beast, and how do you pull it off? How do avoid detection? How do you disperse the disease?
^note, these aren't multiple questions. They're all one and the same. Just different aspects, that's all.