In the ending to H.G. Wells War of the Worlds, the invading Martians are killed off by microbial infections they don’t possess immunity to. Instinctively, this makes sense. We've seen in our own history how certain populations have nearly died off when they have come into contact with new pathogens they don’t possess immunity to.
What the scenario in War of the Worlds overlooks is the slim possibility an Earth born pathogen would be able to infect an alien species. Bacteria and viruses infect us by hijacking the biochemical processes of our cells and using them for their own replication. These pathogens would be unable to do the same thing in an alien organism that contained just the slightest deviation in its biochemistry.
To explain why Earth pathogens would be unable to infect alien organisms, it is necessary to give an overview of the basic building blocks of life. Our genetic information is carried in DNA, deoxyribonucleic acid. DNA consists of two linked strands of nucleotides. Each nucleotide is composed of a sugar residue, a phosphoryl group, and a base. There are four different bases used in our DNA: adenine (A), cytosine (C), guanosine (G), and thymine (T). Our genetic information is contained in the sequence of these four bases.
Many of the biochemical processes of our cells are carried out by proteins, molecules that have a unique sequence of their own. Proteins are large chains of amino acids, and there are twenty different amino acids that can be incorporated into them. The information for the amino acid sequences of our proteins is contained within the base sequence of our DNA. A three base sequence, termed a codon, signifies an amino acid. For example, the base sequence ACG is the codon for the amino acid threonine, while the sequence CGT is the codon for arginine. Accuracy is important as a single mistake in the amino acid sequence could render a protein non-functional.
The viruses and bacteria that infect our bodies make use of the same processes mentioned above. Their genetic information is contained in DNA, which encodes the sequential information for their proteins, using the same basic building blocks. Pathogens would not be able to replicate inside of our cells if their DNA was composed of different bases, or if their genetic code differed from ours, or if the pathogen needed a different pool of amino acids from the one we use.
I believe there’s a good chance any alien life would have a biochemistry vastly different from ours. Even if their genetic information was carried in DNA, they might use different bases, or have a different genetic code, or use different amino acids in their proteins; amino acids our bodies don’t produce.
In my series, the Lifespan Wars, the enemies of the series, the Hozans, make heavy use of genetically engineered pathogens. Because of the differences in biochemistry among the twelve sentient species, each pathogen can infect one species, and only one species. As humans have an unknown biochemistry to the Hozans, they are unable to construct any bio-weapons to infect them. This natural immunity has led the Domarians to employ humans in their war effort.