That appears to be an Oleander Hawk-moth a member of the Sphinx moth family.
I think the only other possibility for Taiwan is the quite similar Jade Hawk-moth, but that supposedly has a dark purplish colored head.
Image for comparison:
Source: Shantanu Kuveskar / CC BY-SA.
Those appear to be the eggs of a species of apple snail in the genus Pomacea, probably the introduced invasive golden apple snail (Pomacea canaliculata), but there are at least two other candidates1,2.
You can learn more about this species and the problems it causes from this Columbia University webpage.
For comparison — Figure 3 from "The identity, ...
I am pretty sure that this is an oil beetle (also known as blister beetle) of the family Meloidae. In this special case this is probably the violet oil beetle (Meloe violaceus), although this does not completely coincide with the size you mention. The slightly different color is probably caused by the different lighting.
See the image from this Wikipedia ...
That actually looks like some sort of caddisfly larvae in its case to me.
Are you sure that's it's 'body' & not a case?
Did you find it in water?
Or a Case Bearing Clothes Moth (Caddisfly are closely related to moths).
This one (a clothes moth larvae) looks very similar to your image.
There are plenty of diseases that affect insects, Dicistroviridae is one.
There are also plenty of fungal & bacterial ones, it's just that they aren't generally of concern to us so study of & information on them is somewhat light.
If you Google 'insect disease' you'll likely just get links for human diseases they're vectors for.
If you want to ...