Something like milk that is ingested orally seems an unlikely vector for biowarfare. The role of gastric juice as an antimicrobial defense is widely assumed because of its hydrochloric acid content. However, a 2005 review article summarized the literature about viruses, saying, “The role of gastric HCl in the defence against viral infection is not known.” (Tom C. Martinsen, et al., Gastric Juice: A Barrier Against Infectious Diseases, Basic and Clin. Pharmacol. and Toxicol., 96: 94-102, 2005.) Are the viruses causing the wide array of chronic illnesses in humans, resistant to degradation during transit through the stomach?

Unfortunately, this is the case. So at first blush, a naïve observer might question your pronouncement that milk is routinely used as a vector for virus delivery orally to hapless humans, thinking that in most cases the virus would not survive the acidic environment which is designed, after all, as a barrier to ingesting living organisms that might be pathogenic. But the fact that nature is designed that way does not mean it is a perfect system or that it cannot be surmounted. This, the aliens have done in cultivating viral strains that are sufficiently acid resistant to survive in meaningful numbers and seed the body nonetheless. There is some attrition, though it is a function of dose in terms of the number of viral particles as well as what might be ingested along with the milk affecting residence time in the stomach and pH levels maintained, and so on. But keep in mind, the aliens do not need a perfect delivery each and every outing, because this is something to which people are exposed on a lifelong basis, in many cases at regular intervals. And once the virus takes up residence in the body, the deed is done, likely for life, because susceptible individuals will never recover and clear the virus on their own.