There are plenty of bacteria that metabolize compounds that are not typically important in what we consider organic life on the surface of Earth.

If you're looking for a purely physical (non-software) "bug" to infect your androids, why not an actual bug - a type of bacteria that feeds on a specific chemical compound found in androids but not typically in organic life? (silicon?)

There are plenty of bacteria that metabolize compounds that may not immediately seem related to organic life on the surface of Earth.

If you're looking for a purely physical (non-software) "bug" to infect your androids, why not an actual bug - a type of bacteria that feeds on or processes a specific chemical compound important to androids but not typically important to sentient organic life? The bacteria could "infect" and then destroy a computer chip or battery, and it could "spread" just like any other bacteria.

Examples of bacteria which are more or less harmless to humans but process things we wouldn't expect them to, things which may be important to an android:

Bonding silicon and carbon: https://www.newscientist.com/article/2114054-bacteria-taught-to-bond-carbon-and-silicon-for-the-first-time/

Mineralizing arsenic and barium: https://www.nature.com/articles/ncomms14393?WT.feed_name=subjects_biochemistry

Bacteria being used to remove lithium (think: it eats batteries!) https://www.ncbi.nlm.nih.gov/pubmed/16384797