The slow accumulation of genetic changes to an influenza virus over time is referred to as

Antigenic drift refers to the slow accumulation of genetic changes to an influenza virus over time.

Explanation:

Antigenic drift is defined as the mechanism by which viruses undergo variation. This mechanism involves the slow accumulation of mutations in the viral genes, that are responsible for coding the antibody binding sites. This leads to the formation of a new strain of virus, which can't be inhibited by the old antibodies. Due to this, the virus can easily spread the disease.

The antigenic drift occurs in the influenza A virus and also the influenza B viruses.

Therefore, Antigenic drift refers to the slow accumulation of genetic changes to an influenza virus over time.          

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.