Grineer Cloning Decay Syndrome

It's explained on the wiki (spoiler warnings, obviously):

Cloning Decay Syndrome is a disease that affects, as the name implies, Grineer clones. Responsible for the decaying physical state the faction as a whole exhibits.

It's the problem you get when you make a copy of a copy of a copy of a copy - minor artifacts compound to become major flaws, and after several generations of what amounts to extreme inbreeding the Grineer are suffering from sterility and severe decay.

Grineer tend to suffer from major organ failure fairly early in life. If they're highly ranked enough to be deemed worth maintaining, they're fitted with cybernetic augmentations to replace the decaying organs' function. If not, well, there's sixty more clones cooking in the next batch, recycle this one into nutrients...

Conjecture is that the problem is actually Working-As-Designed -- the Orokin (who created them as a builder/labourer subrace) are repeatedly on record as building "flaws" into their various servitors, possibly to make them more easily contained should they turn on their rightful masters.

Various high-ranking Grineer are involved in projects that they hope will resolve this:

General Vor started "studying" (IE, capturing and vivisecting) Tenno in an attempt to use "their divine blood" to cure CDS, in direct contravention of the Twin Queen's orders (who just wanted them destroyed ASAP). It didn't work and he wound up demoted to captain and later cut in half for his pains.

Further out in the Solar System,

Tyl Regor operates massive genetic laboratories and archaeological excavations in the deep oceans of Uranus, hoping to find a cure through science. He's had several partial successes which make your life difficult, and his forces are definitely tougher than inner-system Grineer, so be careful when venturing there.

The Twin Queens have their own plan for dealing with the problem, but that is extra double spoilers. Enjoy playing.