Organic debris from comets have started life on earth, dominated evolution and are responsible for the waves of disease which sweep the planet. Organic compounds have been discovered in the tail of Halley's Comet some of which are about the size of viruses and others of which could be desiccated bacteria. As the earth passes through the tail of a comet some of the material penetrates the atmosphere. Particles the size of bacteria may take a few weeks to fall through the air, particles the size of viruses may take 10 years or more to descend.
More serious, long-lived viruses like smallpox and now the Aids virus HTLV III, originated from space then spread from person to person. Smallpox could keep going for 200-300 years once it had arrived. Aids began in a shower of organic material on central Africa 10 years ago.