When it comes to the Corona virus, I doubt rain will help. In a direct approach of this, you should think that rain washes everything away, and falling water also causes suction, so it's not unthinkable some viruses will wash away.
But, rain also causes moisture in the air, and that is precisely what the corona virus needs to survive outside somebody's body. I do not know if the virus can move up in the air, or be lifted by water vapor, but if so, rain can work counterproductive.
So basically there are two theories about this. Most flu variants and corona variants appear not to be able to live long in dry areas, which could explain why most African countries have very little to no infections at all.