Porcine reproductive and respiratory syndrome virus (PRRSV) is believed to be highly infectious because of the rapid spread of the virus through populations of domestic swine throughout the world. However, no information is available on the minimum infectious dose of PRRSV. In our experiment, 10 groups of pigs were inoculated with five different quantities of PRRSV via two different routes: intramuscular and intranasal. The presence of virus in serum and early immune response of pigs were monitored for 21 days. We found that 10 or fewer virions were sufficient to achieve infection. No significant difference in the level of immune response of pigs to PRRSV infection was observed among different treatment groups. However, intramuscular exposure appeared to induce a more uniform immune response compared to intranasal exposure. These results confirmed that PRRSV is highly infectious; a fact that should be taken into consideration when designing strategies for control of PRRSV.