The meaning of ‘absent from controls’ in passage ❶ and ‘but not in controls’ in passage ❷

The meaning of ‘absent from controls’ in passage ? and ‘but not in controls’ in passage ?

The passage below is from Jellyfish age backwards by Nicklas Brendborg.

? A study from Taiwan provides the main suspect. The Taiwanese researchers discovered that people infected with the herpes virus are two-and-a-half times more likely to get Alzheimer’s than those who aren’t –that is, unless they are taking anti-herpes medication. This medication suppresses the virus, and interestingly, it also brings the risk of Alzheimer’s back to normal. The case strengthens as several research groups have found traces of the herpes virus in brain tissue samples from deceased Alzheimer’s patients (while being absent from controls). In one study, the virus was even found inside the amyloid beta clumps in Alzheimer’s brains. Researchers can also duplicate the effect in the laboratory. If brain cells in culture are infected with herpes virus, amyloid beta clumps appear – unless anti-herpes medicine is also added.

? Further down the list of suspects are the bacterium Chlamydophila pneumoniae (not to be confused with the sexually transmitted infection) and fungi such as Candida albicans. Again, both have been discovered in the brains of deceased Alzheimer’s patients but not in controls. At this point, the best evidence is for the herpes virus, but as we’ve discussed already, one-two combinations from microbes are not uncommon. The culprit could be a single microbe with the rest being simple followers, it could be a combination, or microbes could turn out not to be the culprit after all. We don’t yet know, but given that Alzheimer’s is currently untreatable, it won’t hurt to take the microbial theory seriously.

I have questions about the underlined ‘absent from controls’ in passage ? and ‘but not in controls’ in passage ?.

Above all, I want to define the meaning of the ‘control’. It seems to mean ‘one (such as an organism, culture, or group) that is part of a control experiment and is used as a standard of comparison’ as explained in Merriam-Webster Dictionary.

(Am I right?)

Then ‘absent from controls’ in passage ? seems to mean the study was done without controls. (Am I right?)

And ‘but not in controls’ in passage ? seems to suggest the study was done with controls. (Am I right?) Then the sentence in question can be rewritten as “both bacteria have been found in the brains of deceased Alzheimer’s patients but not been found in controls”. (Am I right?) Then ‘controls’ in this passage seems to mean ‘deceased patients without Alzheimer’s disease’. (Am I right?)

Thanks in advance.