This paper discusses the following ten subtitles with the contents indicated. 1. To meet a microbe: discusses the four alternatives in host-microbe interactions. 2. Receptors and signal transduction giving gene activation: discusses the lipopolysaccharide receptor and the limitations of cell cultures versus use of live animals. 3. Effector molecules--antimicrobial peptides with and without cysteines. A data base exists with over 500 sequences. This paper gives a general overview of five classes of gene-encoded effector molecules, based on the absence or presence of cysteines. These molecules are peptide antibiotics with wide spectra against different microbes. They are synthesized as propeptides and post-translational modifications are common. 4. Effectors of innate immunity--lethal action without host damage: evaluates current opinions about the mode of action of peptide antibiotics and the fact that these effectors do not create host damage. 5. Genes, introns and movable elements. Two cecropin genes containing movable elements and the human cathelicidin gene for proFALL-39/hCAP18 are discussed. 6. The natural microflora. Hippos or frogs as model systems. This section includes the isolation of bacteria from the normal flora of frogs; Aeromonas hydrophila, the bacterium found on all five frog species studied; arguments and selected examples of frog-microbe interactions in vivo and in vitro; and the use of glucocorticoids as control for nuclear factor-kappa B/I kappa B alpha regulation of effector genes. 7. The use of germ-free mice--hard facts from hard work: summarizes new findings which indicate that germ-free mice are born with a set of antibacterial peptides in their small intestine. The intestine of germ-free mice monoinfected with A. hydrophila have peptide patterns that differ depending on a pretreatment with cortisone. 8. Looking back--an evolutionary perspective on innate immunity: arguments for an early evolutionary need for gene-encoded antibacterial factors. Caenorhabditis elegans should provide some answers. The finding of cecropin-like peptides in Helicobacter pylori and the indications that cecropins are derived from ribosomal protein L1. 9. What about viruses? Arguments for the lack of innate immunity against viruses. 10. Five questions floating in the pond of immunology. The normal microflora, its size and control are too often left out from immunological thinking. Animal model systems may sometimes invite misinterpretation. Which animal species are more equal than others?