FIDEL CASTRO got off an especially nasty blast at Uncle Sam the other day to celebrate his revolution's 28th birthday. Noting that assorted plagues had hit the crops and the population of Cuba, he raised the possibility that the CIA was responsible for an outbreak of dengue fever that has infected several hundred thousand Cubans and killed more than 100 people. Presumably he meant to trade on whatever susceptibility to such verbal germ warfare exists in Cuba and elsewhere in the Caribbean, and to deflect attention from his own performance in a field, public health, where his regime is accustomed to making broad claims of service to the people. Not a shred of evidence has been produced to support his charge, which the U.S. government promptly denied.

There is more to it, however, than another round in the intensified slanging match that Cuba and the United States have conducted since Ronald Reagan became president. It so happens that even before Mr. Castro spoke, the State Department had approved a Cuban request to ship a supply of a pesticide intended to kill the mosquitoes that spread dengue fever. You can say that the United States has its own interest in confining the outbreak of any disease that might run through the region.But is is also true that the release of the pesticide is a disinterested humanitarian gesture, one of the few this administration has made to Cuba.

American specialists report, further, that Havana has rebuffed their efforts to get the opportunity to examine the Cuban dengue fever epidemic close up. These specialists could conceivably help ensure that this deadly and dangerous plague does not spread further inside or outside Cuba. What kind of narrow political calculus is it that leads Fidel Castro to impede efforts to fight this disease? Who is responsible for "germ warfare"?